The battle of Saipan (15 June-9 July 1944) was the first invasion of the Marianas campaign, and it took nearly a month for US forces to secure the fairly small island.
Saipan was the base of the Japanese Central Pacific Area Fleet, a fairly new command that had been created from the remnants of the forces defeated in the Marshall Islands and Caroline Islands. It was commanded by Admiral Nagumo Chuichi, the commander of the First Air Fleet during the attack on Pearl Harbor. The island was also defended by General Obata Hideyoshi's 31st Japanese Army, which had 30,000 troops and 48 tanks on Saipan (Obata was actually away from Saipan, visiting the Palau Islands when the invasion took place, and command was actually held by General Saito of the 43rd Division). There was a working airfield at Aslito, at the southern end of the island, and a seaplane base at Tanapag Harbour. The Japanese had spent a great deal of effort and money building fortifications on Saipan, including twelve naval observation posts, four gun positions, barracks, air raid shelters. The Navy was officially in charge, but that appears to have been largely ignored by the Army. Many of the troops arrived just before the invasion, and others arrived without most of their supplies after their transport ships were torpedoed by the Americans. The Japanese plan was to defend the beaches, and try and destroy the invasion before it could get established. If that failed, then counterattacks were to be launched to complete the job.
The attack was carried out by General Holland Smith's 5 Amphibious Corps.
Saipan had a limited number of potential landing sites. On the west coast the beaches sloped up gently to the shore, but the coast was protected by reefs that ran along most of the island. The other shores were mainly free of reefs, but also only had small beaches leading to steep slopes, limiting the potential for a landing. On the west coast there was a gap in the reef off Charan Kanoa (towards the southern end of the west coast) and a dredged channel leading to Tanapag Harbor just to the north of the centre of the west coast. There were also very few harbours - Tanapag Bay was partly sheltered against most winds, while the anchorage at Garapan was only sheltered to the east.
The island was long and narrow, with two peninsulas on the east coast on either side of Magicienne Bay on the south-eastern corner of the island. The Kagman Peninsula, to the north of the bay was the larger of the two. There was an airfield at Aslito at the southern end of the island. The central and northern areas of the island are rather mountainous. The island is dominated by Mount Tapotchau in the centre, and the mountains run north to Mount Marpi.
The initial invasion was to involve two Marine divisions, both of which were to land on the southern part of the west coast. The 4th Marine Division was to land on the right, south of Afetna Point, from the town of Charan Kanoa south towards the south-western tip of the island (Blue and Yellow Beaches). The 2nd Marine Division was to land on the left, running north from Afetna Point almost to Garapan (on Green and Red Beaches). Further to the north a naval force carrying the reserve regiments from both divisions would carry out a diversionary demonstration around Tanapag Habor, doing everything apart from actually landing. Both divisions were to push inland - the 4th to capture Aslito airfield, the 2nd to take Mount Tapotchau and nearby heights. The 27th Division was the corps reserve, and had to plan for a wide range of options (none of which actually came to pass).
The invasion was to be covered by a massive naval bombardment, originally planned to involve 7 fast battleship, 4 older battleships, 2 heavy and 3 light cruisers, 15 destroyers and 24 LCI gunboats. Another 33 ships were to bombard Tinian at the same time. The fast battleships and destroyers were to attack on D-2 to knock out aircraft, airfields, coastal defence and anti-aircraft guns and set fire to the sugar cane fields near the beaches. D-1 would see the older battleships and cruisers join in. On D-Day the bombardment would increase in intensity, with beach defences coming under particular attack. The LCIs would advance towards the beaches ahead of the first wave of amphibian tanks. Air support would come from the fast carrier task force.
The landings themselves would be lead by LVT (A)s armed with 75mm howitzers or 37mm guns. They would shield the LVTs carrying troops, and then support the actual fighting on land.
Task Force 58 arrived in the attack area a day earlier than expected, and Mitscher received permission to launch an extra attack on 11 June. This fighter sweep caught the Japanese by surprise, and destroyed something between 147 and 215 aircraft on the islands. However this wasn't the first air raid to hit Saipan, and the Japanese didn't realise that an invasion was imminent. This must have become clear over the next four days, as Mitscher's carrier groups carried out three days of concerted attacks on the Japanese defences of the Marianas.
The naval bombardment began on 13 June, when seven fast battleships and eleven destroyers were detached under the command of Admiral William A. Lee Jr. This force bombarded Tinian and Saipan between 1040 and 1725, probably with little impact.
The main bombardment force - seven old battleships, eleven cruisers and twenty-six destroyers, under Rear Admiral Jesse B. Oldendorf, arrived on 14 June. These ships had far more training and experience of shore bombardment, and were also allowed closer to shore, and they were thus more effective than the fast battleships. Many anti-aircraft guns were destroyed, but the fixed fortifications mainly survived intact.
The invasion fleet moved into place just after 0500 on 15 June. The pre-invasion bombardment began at 0530. The Japanese High Command reported that it wasn't very effective, but some of the troops on the ground thought otherwise. As the real invasion prepared to move off, a diversionary force of Marines carried out a fake landing off Garapan, reaching within 5,000 yards of the shore, but the Japanese didn’t fall for the ruse. The naval bombardment paused between 0700 and 0730 to allow for an air strike, then resumed. The landing was delayed until 0840, meaning that the advance was to begin at around 0810.
In the north the 2nd Marine Division (General Thomas Watson) landed with four battalion teams in a row. The division landed in four waves. The first was made up of alternating lines of troop carrying LVTs and gun carrying LVT(A)(4)s with 75mm howitzers. The remaining three waves were made up of just LVTs.
In the south the 4th Marine Division sent in the amphibian tanks first - 68 LVT(A)(1)s and 16 LVT(A)(4)s. Four waves of troop carrying LVTs then followed.
As the LVTs crossed the reef the Japanese opened fire, but with little impact. On the 2nd Division front only seven LVTs of both types were knocked out by enemy fire and two by the rough seas, meaning around 98% reached the shore safely. However their complex formation broke down, and the formation drifted north, causing overcrowding in the centre and opening up a bigger than wanted gap between the two divisions. The landings began at 0843 and were over by 0908.
On the right the 4th Division things went more smoothly. Three of the LVT(A)s were knocked out, and only two of the 196 troop carrying LVTs. The landings lasted from 0843 to 0907.
Once the LVTs were on shore the plan to have them advance some way inland, acting as conventional tanks, was largely unsuccessful. They struggled to cross any difficult ground and were very vulnerable to Japanese fire. As a result the troops quickly got out and fought as conventional infantry. On the 2nd Division front the advance slowed to a crawl as the LVTs became badly mixed up, and struggled to get past a wooded bank. The 4th Division had more luck, and was able to advance through Charan Kanoa on their left. Further south less progress was made, although some did reach Agingan Point before being forced to withdraw by US naval gunfire.
At first the Marines struggled to get far inland, but as the day wore on the first tanks and artillery guns were able to land. General Watson moved onshore on D-Day. The 2nd Marines were estimated to have lost 238 dead, 1,022 wounded and 315 missing on the first day. On the right the 4th Marines made more progress on the left than the right. General Schmidt also came ashore on D-Day, although probably too soon. Overnight the left wing was ordered to pull back to confirm with the rest of the line, to avoid the risk of a counterattack against their exposed position. The beachhead was now 10,000 yards wide and around 1,000 yards deep in most places, so although few of the final objectives for the day had been captured, the landings had been a general success.
As expected, the Japanese planned a counterattack for the night of 15-16 June. This began with a strong attack on the 6th Marines, 2nd Division, on the exposed left flank of the beachhead, which began at around 2000. This was the first of three attacks on this flank, all of which were repulsed with the help of naval star shells and eventually five medium tanks. These attacks cost the Japanese at least 700 dead. The 4th Division was also attacked, but these were less organised, and posed less danger.
Early on 16 June Admiral Spruance received news that a massive Japanese naval build-up was taking place, and an attack was likely. The Japanese had decided to seek out their long awaited decisive battle in the Marianas, and were preparing for a massive carrier attack on the US fleet. Most of the aircraft involved were to land on the islands, allowing the attack to be carried out from very long range. This news forced Spruance to postpone the invasion of Guam, which had been set for 15 June. The threat of a major naval battle forced Spruance to order the unloading plans to be altered - the transport ships would retire to the safe transport area at the end of 17 June and only those ships that were urgently required would return on 18 June. The amphibious forces under Admiral Conolly would withdraw to the east. Some of the cruisers and destroyers allocated to the invasion forces were transferred to Mitscher's control for the upcoming battle. One other result of this news was the decision to land the 27th Division, the reserve unit for the invasion, as soon as possible.
On 16 June the Marines continued to expand their beachhead. The 2nd Marines pushed across Afetna Point and made contact with the left flank of the 4th Marines. The 4th Marines managed to take Agingan Point, securing their right flank, but the advance towards Aslito Field went less well.
General Saito ordered a second counterattack on the night of 16-17 June, but he didn't give his men enough time to prepare. Eventually a combined armoured and infantry assault was launched at about 0330 on 17 June, with 37 tanks and around 1,000 infantry, but this was driven off by the 6th Marines, and by the time the attack ended the Japanese had lost around at least 24 of the tanks and an unknown number of infantry.
On the same night the 27th Infantry (General Ralph Smith) landed on the 4th Marine Division's beaches. Part of the division was immediately allocated to a 4th Marine attack towards Aslito airfield, which was due to start at 0730 on 17 June. Despite heavy resistance this attack got very close to the airfield. Elsewhere the Americans were advance from the northern beachhead as well.
The airfield was secured on 18 June, and only five days later, on 23 June, it was being used by P-47 Thunderbolts. The Americans also reached the east coast of the island around Magicienne Bay. On the same day the Japanese attempted to attack the American beaches from the sea, using a force of 35 small boats, but this force was intercepted and destroyed. The Japanese were forced to plan a new defensive line, which would run across the width of the island to the north of the beachhead.
The most important events of 19 June took place in the skies to the west of the islands (battle of the Philippine Sea). The Japanese launched a series of heavy air attacks, hoping to knock out the American fleet, but the attack turned into a disaster for the Japanese. Three quarters of the aircraft involved were lost - some 330 out of 430 - and the carriers Shokaku and Taiho were sunk by submarines. On the following day Mitscher attempted to catch the retreating Japanese fleet, destroying another 65 aircraft and sinking the Hiyo. Although around 100 American aircraft were lost, mainly because they ran out of fuel on the way home, most of their crews were rescued. The Americans had destroyed Japanese naval air power, but because the carriers got away they didn't realise they'd done it!
On 19 June the Americans began to pivot to the left ready to advance north up the island. The 2nd Marines were to stay roughly where they were. The 4th Marines were to move north along the east coast to form up to their east. The 27th Infantry was to focus on clearing out Nafutan Point, the south-eastern corner of the island, to secure the airfield. This would actually take longer than expected, and the area wasn't entirely secured until 27 June (by which time the advance north had already begun).
On 21 June General Holland Smith decided to withdraw most of the 27th Division from the fight at Nafutan, ready to act as the corps reserve for the advance north. One battalion was to remain at Nafutan, while the rest of the division was withdrawn. General Ralph Smith managed to convince him to leave two battalions to deal with the resistance. The plans for the point kept altering, and the confusion helped break the relationship between the two Smiths. Eventually the last resistance was cleared by 27 June.
Meanwhile the Marines were preparing for their advance north. They had reached their jumping off line by the end of 20 June, the same day that the 106th Infantry, the last element of the 27th Division, finally landed on the island. The Japanese had also formed a new line across the island, but they had already lost about half of their strength. The attack began on 22 June, with the 2nd on the left and the 4th on the right. The bulk of the 27th Division was prepared to move into the middle of the line if a gap developed. By the end of 22 June the 27th was ordered to prepare to take over the area occupied by the left wing of the 4th Marines.
The 27th ran into fierce resistance in the centre of the line, and made little progress on 23 June. This angered General Holland Smith, who blamed the Army for the slow progress of the entire attack, but also probably under-estimated the difficulties of their task. On 24 June the attack resumed, and once again the 27th made limited progress. On the flanks the Marines did better, against lighter resistance. The 2nd Marines reached the outskirts of Garapan, the 4th Marines had captured the Kagman Peninsula.
Although the 27th felt that they were facing the heaviest resistance, over the previous two days they had also suffered the lowest casualties - 277 compared to 333 for the 2nd Marines and 812 for the 4th. By the end of 24 June General Holland Smith decided to relief Ralph Smith, and replaced him with General Sanderford Jarman, who was serving as the island commander (the senior army officer on the island), until a new commander could arrive from Hawaii. Jarman was eventually replaced by General George W. Griner (at 10.30 on General Griner). The entire affair caused a great deal of bitterness between the Army and the Marines, and most of the senior army officers on Saipan made it clear that they would prefer not to serve under Holland Smith again. The change in command also failed to speed up the offensive, as it took another six days for the key feature on the 27th Division front to be captured.
While the Americans were arguing, the Japanese were rapidly losing strength. Their new defence line had failed, and by 25 June they realised that the battle was being lost. After prolonged pressure, the key positions in the centre of the line began to give way on 27 June, while the 2nd Marines occupied the summit crest of Mount Tapotchau. General Saito decided to establish a third defensive line, across the northern tip of the island. The troops on the second line would carry out a fighting retreat to the new line. The American advance continued, and on 30 June the 27th Division finally managed to close up with the two Marine divisions. Although the army men had a difficult task, their casualty figures were much lower than the Marines - 1,836 since D-Day compared to 4,454 for the 4th Marines and 4,488 for the 2nd Marines. A debate late raged about whether the army division hadn't fought hard enough, or the Marines had suffered unnecessary losses.
As the Americans advanced north, the 2nd Marines focused on the capture of Garapan, while the other two divisions pushed north. The advance was now slow but steady, against the normal desperate Japanese resistance. As the front line narrowed, the 2nd Marines were pinched out and became the corps reserve. By early July it was clear to the Japanese commanders that the island was lost. On 6 July Saito and Nagumo committed suicide, after ordering a banzai charge. The Americans were expecting just such an attack, starting early on 7 July.
On 6 July, with the battle lost, Nagumo and Saito committed suicide. This was followed by a series of suicidal banzai attacks, in which the Japanese lost 2,500 men to no effect. This attack managed to get into a gap between two battalions of the 105th Infantry, and at great cost overran some 105mm guns. The attack was finally stopped short of the 105th's command post. This was an unusually effective banzai charge, but even so the Japanese had only inflicted 917 casualties at the cost of 2,295 dead in the battle with the 105th. A total of 4,311 Japanese dead were counted after the attack.
The last organised resistance came on 8-9 July, and ended with a mass suicide at Marpi Point at the northern end of the island, with civilians andsoldiers taking part.
When the fighting ended a total of 23,811 Japanese dead were counted. Only 736 prisoners were taking, included 438 casuatleis.
The Americans lost 3,225 killed in action, 13,061 wounded and 326 missing. The casualties were split roughly 2-1 in favour of the Marines, about what one would expect from the balance of troops.
Saipan was soon turned into a major US base. Only four months after the conquest of the island, the first B-29s set off for the Japanese Home Islands, at the start of a massive bombing campaign. More immediately Prime Minister Tojo resigned on 18 July, nine days after the end of the battle, along with his cabinet. The attack had also triggered the battle of the Philippine Sea, which stripped the Japanese Navy of its last naval aviators, and meant that the remaining carriers could only be used as a decoy during the battle of Leyte Gulf.
After the fall of Saipan the Americans moved on to attack Guam (21 July-9 August 1944) and Tinian (24 July-31 July 1944), completing the conquest of the main islands in the southern Marianas.