Answer: Isolationism refers to America's longstanding reluctance to become involved in European alliances and wars. Isolationists held the view that America's perspective on the world was different from that of European societies and that America could advance the cause of freedom and democracy by means other than war. American isolationism did not mean disengagement from the world stage. Isolationists were not averse to the idea that the United States should be a world player and even further its territorial, ideological, and economic interests, particularly in the Western Hemisphere.
Nineteenth-century isolationism was based largely on the Monroe Doctrine. This doctrine limited the American sphere and influence to the territorial United States and served to deter European interference in the affairs of the continent. This was a necessary deterrent in an age marked by European empire, especially the Spanish conquest and expansion in Central and South America, Russia declarations regarding areas north of the fifty-first parallel and extending one hundred miles into the Pacific would be off-limits to any foreign powers. The idea that European powers control any part of American soil was abhorrent to American policymakers During the 19th century, America had a strong reluctance to become involved in other countries' alliances and affairs. Isolationists in America argued that the US had a different philosophy than European countries and the US should defend freedom and democracy by not being involved in such things.
During the Spanish-American War, the United States remained isolated and the country fought the war without help and without fighting in Europe. President Roosevelt had the big stick policy, he believed that the US should export its values and become a global power. At the same time, he defended that the US should avoid conflicts. President Roosevelt ended the isolationism in the US and started the modern American philosophy of acting aggressively in foreign affairs even without the support of Congress.