Final answer:
Japan gained control over the Korean Peninsula after the Russo-Japanese War (1904-1905), which occurred due to a rivalry over Manchuria and Korea, marking the emergence of Japan as a global power.
Step-by-step explanation:
Japan's Acquisition of the Korean Peninsula
Japan gained control over the Korean Peninsula following the Russo-Japanese War (1904-1905), a conflict that began due to rival territorial claims over Manchuria and Korea. The victory of Japan in this war marked the first time an Asian nation had defeated a European power in modern history, which demonstrated Japan's growing military and industrial prowess. Prior to this, Japan had already made its mark by winning the First Sino-Japanese War (1894-1895), which led to the Treaty of Shimonoseki that recognized Korea's independence from China, and granted Japan territory, including Taiwan and the Penghu Islands. However, the Russian presence in Manchuria and their growing influence in Northern Korea challenged Japan's interests in the region, eventually leading to the decisive conflict that would affirm Japan's control over the entire Korean Peninsula.
Thus, the correct answer to the student's question is: 4) The Russo-Japanese War.