The Second Northern War (1655-1660) was a conflict primarily involving the Kingdom of Sweden and the Polish-Lithuanian Commonwealth. It began when Sweden invaded Poland, seeking to expand its territory and influence in the region. The war saw significant battles and shifting alliances, with various European powers becoming involved. The war concluded with the Treaty of Oliva in 1660, which established peace between Sweden and Poland. The conflict had lasting effects on the balance of power in Northern Europe, weakening the Polish-Lithuanian Commonwealth while solidifying Sweden's status as a major regional power.