In a bid to recapture territory conceded following the Winter War of 1939-40, Finnish forces cooperated with Nazi Germany and other Axis powers during the invasion of the Soviet Union in June 1941. Rapid Finnish progress in reoccupying lost ground in Karelia during the first few months of the invasion gave way to a more static form of warfare from October 1941. The Finns resisted German pressure to participate fully in the Axis attack on the beleaguered Soviet-held city of Leningrad, and the Continuation War came to be characterized by trench warfare and unconventional operations conducted by both sides behind the front lines. In June 1944 the stalemate was abruptly ended by a massive Soviet offensive that pushed the Finns back; the two sides clashed in a series of major battles, including the battle of Tali-Ihantala, with the Finns halting the Soviet advance before agreeing to an armistice that September.