Complex sentence: In December 1941, Japan attacked the United States in the Philippines and at Pearl Harbor, and the United States declared war on Japan as part of World War II. Japan conquered the Philippines in 1942, in the Philippines Campaign (1941--42).
Split into simpler sentences: In December 1941, Japan attacked the United States at Pearl Harbor and in the Philippines. The United States declared war on Japan and entered World War II. Japan conquered and occupied the Philippines during the Philippines Campaign (1941--42).