The Thirty Years' War (1618-1648) was primarily caused by religious tensions between Protestants and Catholics in the Holy Roman Empire. The conflict began when Protestant nobles opposed the Catholic Habsburg rulers' attempts to impose religious uniformity, leading to the Defenestration of Prague in 1618, where two Catholic officials were thrown out of a window. Political power struggles also fueled the war, as various European powers sought to expand their influence. The involvement of countries like France, Sweden, and Spain transformed the conflict into a broader struggle for dominance, intertwining religious and political motivations throughout Europe.