WW1 the Americans entered 1917, 3 yrs late maybe influential for the last year
WW2 FDR was favourably disposed to helping the Allies with Lend Lease but had to wait for Pearl Harbour before declaring War. We had to accept Eisenhower as SACEUR and McCarthur in the Far East whilst gifting some UK RAF bases to the US as part payment for lend lease
Special Relationship - UK ind nuclear deterent is purchased from US