Different waves of German immigration to the U.S. resulted in German immigrants becoming one of the largest ethnic groups in the US. Starting in the late 1600's, Germans immigrated to the U.S. and established their own communities such as Germantown near Philadelphia in 1683. Germans came to the US for various reasons including economic hardship. Nearly a million Germans immigrated to the U.S. in the aftermath of the German Revolution in the 1840s.
World War I:
At the beginning of the World War I, the U.S. declared its neutrality but soon changed positions after Germany began its unlimited submarine warfare. This phase of the war led to the sinking of various American and European vessels, among them the Lusitania which carried about a thousand passengers including 100 Americans. America officially entered the conflict against the Germans in a war which ended in 1919 with Germany's loss and the signing of the Treaty of Versailles.
Tensions resurfaced when Hitler starting targeting the Jewish population which eventually escalated into the holocaust. Trade agreements between the United States and Germany were eventually revoked and the American ambassador recalled in 1938. However some critics state that, due to the isolationist tendency of the U.S. politics at the time, America did not take sufficient steps to prevent Hitler's rise and the persecution of Jews.
World War II:
As in World War I, the U.S. initially took a neutral position. In the early phase of the war, the U.S. enacted a trade embargo against all the warring nations and this isolationist position did not change until the fall of France and the real prospect of the fall of Britain when the United States began supplying weapons to the anti-German side. Tensions escalated when the United States began sending warships to protect weapon supplies, which eventually fell under attack from German submarines. After Pearl Harbor, the United States officially entered the war which ended with the surrender of Germany in 1945.
The end of World War II saw Germany occupied by France, the United States, the United Kingdom, and the Soviet Union. Eventually the Soviets controlled the eastern German Democratic Republic and the Americans and western allies supported the western Federal Republic of Germany, both established in 1949. Cold war rivalry between the two superpowers dictated the realities in Germany. U.S. aid to Western Germany was characterized by the Marshall Plan, which helped rebuild German infrastructure and economy and provided incentives for Western Germany, among others European countries to remain in the anti-Soviet bloc.
The city of Berlin (in the eastern part of Germany) was also divided between eastern and western powers. The Berlin Wall became a physical symbol of both the Cold War and the Iron Curtain.
Competition between the two German halves remained in place until the collapse of the Soviet Union and the fall of the Berlin Wall in 1989. The reunified of Germany re-established its capital in Berlin.
The Marshall Plan and U.S. troop presence in Germany has left a legacy of cooperation between both nations, politically, economically, and militarily. Although both countries have had recent disagreements on foreign policy, especially with the U.S.-led invasion of Iraq, relations remained favorable overall, especially with the election of pro-American politician Angela Merkel.