The Northern War of 1655, also known as the Second Northern War, was a conflict primarily involving the Swedish Empire and the Polish-Lithuanian Commonwealth. It began when Sweden invaded Poland in 1655, aiming to expand its territory and influence in the region. The war saw significant battles and shifting alliances, with various European powers becoming involved. The conflict resulted in the devastation of Poland and significant territorial gains for Sweden. However, the war also led to a decline in Swedish power over time, as it strained resources and prompted resistance from other nations. The war officially ended in 1660 with the Treaty of Oliva.