The Grand Duke Constantine (1779-1831) was the younger brother of Tsar Alexander I and was his heir until he chose to marry Johanna Grudzinska, a Polish lady.
The Grand Duke was raised by Catherine II. He was personally brave but not a particularly good commander. He fought well at the battle of Novi (15 August 1799) and was given the title Tsarevich by his father Paul I despite only being second in line to the throne (behind his brother Alexander).
In March 1801 Constantine's father Paul I was murdered and his older brother became Tsar Alexander I. Constantine remained in the army, despite his new status as heir to the throne. He commanded the Guard Corps at Austerlitz, where he was nearly killed by a Mameluke in French service.
He was a supporter of the alliance with France, but commanded V Corps during the campaign of 1812 and took part in the campaign of 1813.
He was then appointed Commander-in-Chief of Poland, and was effectively the ruler of the Congress Kingdom of Poland. His marriage to Johanna Grudzinska meant that he had to surrender his position as heir to the throne in favour of his younger brother Nicholas.
During the Decembrist revolt in 1825 the rebels wanted to place Constantine on the throne in place of Nicholas, but he refused to support them.
In 1830 Poland rose in revolt, much to Constantine's surprise. He failed to put down the revolt and died of cholera at Vitebsk in June 1831.