As Austria, France and Russia formed a new anti-Prussian coalition, Frederick became convinced that Prussia would be attacked in early 1757 and once again chose to strike first. On 29 August 1756 he preemptively invaded neighbouring Saxony, beginning the Third Silesian War. As Austria's and Prussia's allies joined the fighting, the conflict quickly widened into what became the pan-European Seven Years' War. The Prussians occupied Saxony in late 1756 and made large advances in Bohemia in early 1757, winning a series of battles while advancing to Prague. In May Prussian forces drove back the Austrian defenders in the Battle of Prague, taking great losses, and then besieged the city. An Austrian counter-attack culminated in the major Austrian victory at the Battle of Kolín on 18 June, which drove the Prussians out of Bohemia entirely. Meanwhile, Russian and Swedish invasions from the east and north divided Prussia's forces. The Russian invaders in East Prussia won the Battle of Gross-Jägersdorf on 30 August, but they made little strategic progress due to recurring logistical problems.