Final answer:
The United States became more active in international affairs from 1872 to 1917 due to the closure of the western frontier, the rise of imperialism, the need for new markets for industrial goods, and the establishment of a modern naval presence.
Step-by-step explanation:
Between 1872 and 1917, the United States became increasingly active in international affairs due to several significant factors. The closing of the American frontier marked a turning point, as there was no longer western land to expand into, leading Americans to look outward. Imperialism became a driving force, as the U.S. observed European powers expanding their empires and did not want to fall behind.
America's industrial production had increased, necessitating new markets for goods. Additionally, a military presence, specifically a modern navy, was recognized as vital to become an international power. The Spanish-American War of 1898, where the U.S. acquired territories such as Puerto Rico, Guam, and the Philippines, and the Open Door Policy in China are examples of the U.S. asserting itself globally. These actions showed a shift from isolationism towards a more interventionist stance in world affairs, reflecting the country's growing international interests and ambitions.