During both World War I and World War II, Britain employed several strategies to encourage U.S. participation in the war.
In World War I, Britain used propaganda to sway American public opinion against Germany. They highlighted German atrocities in Belgium and the sinking of passenger ships like the Lusitania by German submarines. The British also cut the transatlantic telegraph cable from Germany to the U.S., which meant that most war news in America was sourced from Britain and thus had a pro-British bias.
In addition, Britain appealed to shared cultural and linguistic ties with America, emphasizing a common heritage and democratic values. They also stressed the economic implications of a German victory for the U.S., as many American banks had lent significant sums of money to the Allies.
During World War II, Britain again used propaganda to highlight German aggression and atrocities. Prime Minister Winston Churchill also worked closely with President Franklin D. Roosevelt, building a personal relationship that helped draw the U.S. into the war.
The British government shared intelligence with their American counterparts, including information about German military plans obtained through code-breaking (the Ultra secret). This not only helped convince American leaders of the Nazi threat but also facilitated joint military planning once America entered the war.
Finally, Britain encouraged American involvement through policy measures such as the Lend-Lease Act, which allowed America to supply military aid to foreign nations during World War II. This act was initially aimed at helping Britain resist Nazi Germany and was later extended to other Allied nations. It effectively ended American neutrality in the conflict without formally entering the war until after Pearl Harbor attack by Japan in December 1941.