The primary cause of the Russo-Japanese War (1904-1905) was the rival imperial ambitions of Russia and Japan over territories in East Asia, particularly Manchuria and Korea. Japan sought to expand its influence in the region, while Russia aimed to secure access to warm-water ports and control of the Korean Peninsula. Tensions escalated as both nations reinforced their military presence and interests, leading to conflict when Japan launched a surprise attack on Russian naval forces at Port Arthur. The war ultimately marked Japan's emergence as a significant power on the global stage and highlighted Russia's weaknesses.