Complex sentence: From 1939 until his death in December 1940 he was Ambassador to the United States, Proving highly successful in winning Americans support for these British war effort, most notably the Lend Lease program, which passed Congress after his death.
Split into simpler sentences: From 1939 until his death in December 1940 he was Ambassador to the United States. He proved highly successful in winning America's support for the British war effort, most notably the Lend Lease program, which passed Congress after his death.