[Blake#1]

I will give you a short answer to your request. Leaders use their people to achieve their political, military, etc. aims. Roosevelt aim was to get the USofA into WWII on England's side. It worked.

[jamaksin]

Or, another theory - to save Stalin.

Stalin got far more Lend-Lease aid than England or China - for example, tens of tons of nuclear materials were sent in 1943 (the Trinity Test took place in 1945).

But then, Harry (the Hop) Hopkins was Soviet agent 19 )known from the Venona Project decrypts).