The "rock-solid" relationship between the United States and Great Britain that President Barack Obama described during his March 2012 meetings with British Prime Minister David Cameron was, in part, forged in the fires of World Wars I and II. Despite fervent wishes to remain neutral in both conflicts, the U.S. allied with Great Britain both times.
World War I
World War I erupted in August 1914, the result of long-standing European imperial grievances and arms races. The United States sought neutrality in the war, having just experienced its own brush with imperialism that included the Spanish-American War, 1898, (of which Great Britain approved), and the disastrous Filipino Insurrection that soured Americans on further foreign entanglements.
Nevertheless, the United States expected neutral trade rights; that is, it wanted to trade with belligerents on both sides of the war, including Great Britain and Germany. Both of those countries opposed the American policy, but while Great Britain would stop and board U.S. ships suspected of carrying goods to Germany, German submarines took the more dire action of sinking American merchant ships.
After 128 Americans died when a German U-Boat sank the British luxury liner Lusitania (surreptitiously hauling weapons in its hold), U.S. President Woodrow Wilson and his Secretary of State William Jennings Bryan successfully got Germany to agree to a policy of "restricted" submarine warfare. Incredibly, that meant a sub had to signal a targeted ship that it was about to torpedo it so that personnel could debark the vessel.
In early 1917, however, Germany renounced restricted sub warfare and returned to "unrestricted" sub warfare. By now, American merchants were showing an unabashed bias toward Great Britain, and the British rightly feared renewed German sub attacks would cripple their trans-Atlantic supply lines.
Great Britain actively courted the United States -- with its manpower and industrial might -- to enter the war as an ally. When British intelligence intercepted a telegram from Germany's Foreign Secretary Arthur Zimmerman to Mexico encouraging Mexico to ally with Germany and create a diversionary war on America's southwestern border, they quickly notified Americans. The Zimmerman Telegram was genuine, although at first glance it seems like something British propagandists might fabricate to get the U.S. in the war. The telegram, combined with Germany's unrestricted sub warfare, was the tipping point for the United States. It declared war on Germany in April 1917.
The U.S. enacted a Selective Service Act, and by Spring 1918 had enough soldiers in France to help England and France turn back a massive German offensive. In Fall 1918, under the command of General John J. "Blackjack" Pershing, American troops flanked the German lines while British and French troops held the German front in place. The Meuse-Argonne Offensive forced Germany to surrender.
Treaty of Versailles
Compared to France, Great Britain and the United States took moderate stances at the post-war treaty talks in Versailles, France. France, having survived two German invasions in the last 50 years, wanted severe punishments for Germany, including the signing of a "war guilt clause" and the payment of onerous reparations. The U.S. and Britain were not so adamant about the reparations, and in fact the U.S. loaned money to Germany in the 1920s to help with its debt.
However, the U.S. and Great Britain did not agree on everything. President Wilson forwarded his optimistic Fourteen Points as a blueprint for post-war Europe. The plan included an end to imperialism and secret treaties; national self-determination for all countries; and a global organization -- the League of Nations -- to mediate disputes. Great Britain could not accept Wilson's anti-imperialist aims, but it did accept the League, which Americans -- fearing more international involvement -- did not.
Washington Naval Conference
In 1921 and 1922, the U.S. and Great Britain sponsored the first of several naval conferences designed to give them dominance in total tonnage of battleships. The conference also sought to limit a Japanese naval buildup. The conference resulted in a ratio of 5:5:3:1.75:1.75. Simply, for every five tons the U.S. and British had in battleship displacement, Japan could have only three tons, and France and Italy could each have 1.75 tons.
The agreement fell apart in the 1930s when militaristic Japan and fascist Italy disregarded it, even though Great Britain tried to extend the pact.
World War II
When England and France declared war on Germany after its invasion of Poland on September 1, 1939, the United States again tried to remain neutral. When Germany defeated France, then attacked England in the summer of 1940, the resulting Battle of Britain shook the United States out of its isolationism.
The United States began a military draft and started building new military equipment. It also began arming merchant ships to carry goods through the hostile North Atlantic to England (a practice it had abandoned with the policy of Cash and Carry in 1937); traded World War I-era naval destroyers to England in exchange for naval bases; and began the Lend-Lease program. Through Lend-Lease the United States became what President Franklin D. Roosevelt called the "arsenal of democracy," making and supplying materiel of war to Great Britain and others fighting Axis powers.
During World War II, Roosevelt and British Prime Minister Winston Churchill held several personal conferences. They met first off the coast of Newfoundland aboard a navy destroyer in August 1941. There they issued the Atlantic Charter, an agreement in which they outlined the goals of the war.
Of course the U.S. was not officially in the war, but tacitly FDR pledged to do all he could for England short of formal war. When the U.S. officially joined the war after Japan attacked its Pacific Fleet at Pearl Harbor on December 7, 1941, Churchill went to Washington where he spent the holiday season. He talked strategy with FDR in the Arcadia Conference, and he addressed a joint session of the U.S. Congress -- a rare event for a foreign diplomat.
During the war, FDR and Churchill met at the Casablanca Conference in North Africa in early 1943 where they announced the Allied policy of "unconditional surrender" of Axis forces. In 1944 they met at Tehran, Iran, with Josef Stalin, leader of the Soviet Union. There they discussed war strategy and the opening of a second military front in France. In January 1945, with the war winding down, they met at Yalta on the Black Sea where, again with Stalin, they talked about post-war policies and the creation of the United Nations.
During the war, U.S. and Great Britain cooperated in the invasions of North Africa, Sicily, Italy, France and Germany, and several island and naval campaigns in the Pacific. At the war's end, as per an agreement at Yalta, the United States and Britain split the occupation of Germany with France and the Soviet Union. Throughout the war, Great Britain acknowledged that the United States had surpassed it as the world's top power by accepting a command hierarchy that put Americans in supreme command positions in all major theaters of the war.