Since the Revolutionary War, the United States uses diplomacy to gain allies. Because Great Britain had the military advantage, in November 1775, Congress established the Committee of Secret Correspondence to “correspond with friends” . The objective was to retain France and Spain' support (enemies of Great Britain) for independence. Diplomacy was required to end the war. Franklin, John Adams and John Jay (the Peace Commission) negotiated peace In June 1781, and so they reached agreement with the British on November 30, 1782.
After George Washington became president in 1789, in the two terms of his presidency he agreeded that isolation and neutrality were the most beneficial course for the United States, in his inaugural address he spoke of "Peace, commerce and honest friendship with all nations, entangling alliances with none." Before that, in 1781, what today we call Department of State was called Office of Foreign Affairs, both Robert R. Livingston and John Jay ,the first two Secretaries found frustrating the task, in whic they had so little autonomy. The new Constitution of 1789 allotted significantly more responsibility for foreign affairs to the Federal Government.
Despite some deficits in the Department of State, in the first years of the Republic significant triumphs in foreign affairs occured such as the Jay Treaty (1794) and the Pinckney Treaty (1795) regularized relations with England and Spain.
But the reason why the foreign policy was isolationist was that the very foundation of the U.S had in it's principles the idea of avoinding war in the future generations; the world had a different conjecture of war that would end in the 30's. By staying idle to the growing fascim in Europe, (the Johnson Act-1934 and the Neutrality acts -1935 prevented economic or military aid to any country involved in the disputes that were to escalate into World War II) , America encouraged the British in their policy of appeasement and contributed to French paralysis in the face of the growing threat posed by Nazi Germany . After World War II with the creation of the United Nations, isolationism was no longer an option specially to oppose communist expansion-East and West . Negotiating and maintaining peace was the practical responsibility of the new U.N. Security Council, made up of the United States, Great Britain, France, the Soviet Union, and China.