The treaty of Teplitz (9 September 1813) saw Austria formally join the Sixth Coalition, although she had been at war with France since mid-August, and had already fought and lost the major battle of Dresden.
During the Spring Campaign of the War of Liberation Austria had remained neutral. The Prussians and Russians were often outnumbered during the campaign, and suffered defeats at Lutzen and Bautzen, before the fighting was ended by the Armistice of Pleischwitz (4 June 1813).
During the armistice both sides attempted to gain Austrian support. The Austrians were officially allied with Napoleon, and he was married to an Austrian Princess, Marie Louis, but there was no enthusiasm for the alliance. On the other hand many Austrian leaders, probably including Metternich, didn't want to entirely eliminate Napoleon, as they believed this would only benefit Prussia and Russia.
On 26 June Napoleon and Metternich met at Dresden. The meeting didn't go well – Metternich demanded that the French should return all Austrian lands in Italy, end the Confederation of the Rhine, restore Prussia and give Poland to Russia. In return Austria would remain neutral. Napoleon turned down these terms, and on 27 June Austria, Prussia and Russia signed the Convention of Reichenbach, in which Austria agreed to enter the war if Napoleon didn't agree to their terms.
There was one last hope for peace, triggered by news of the French defeat at Vittoria (21 June 1813). Napoleon sent Caulaincourt to Prague (Congress of Prague, 15 July-10 August 1813), but refused to accept any significant compromise.
On 10 August Russia and Prussia notified the French that the armistice would end in seven days. On 12 August Austria officially declared war on France, and Austrian troops made up a large part of the Army of Bohemia, the southern of three armies facing Napoleon. This army advanced into Saxony, ignored the Allied plan not to risk a battle with Napoleon, and suffered a heavy defeat at Dresden (26-27 August 1813), all before Austria had officially joined the Sixth Coalition.
On 9 September 1813 Austria, Prussia and Russia signed the Treaty of Teplitz (then in Bohemia, now in the Czech Republic). All three agreed to restore Austria and Prussia to their borders of 1805 (Prussia never regained most of her Polish provinces). The treaty also called for unity between the Allied forces. The treaty also reaffirmed the Trachenberg Plan (the agreement not to fight Napoleon in person without an overwhelming advantage). Just over a month later the three main Allied armies came together at Leipzig (16-19 October 1813), where they inflicted a heavy defeat on Napoleon, and ended any chance of his hanging on to power in Germany.