Actually, he wanted to join since day one, but he was unable to. The Americans did not want to enter neither WW1 nor WW2. They had to be tricked into it by jewish propaganda which took time and intrigue. But Roosevelt did his best to support the USSR from the very beginning.

As you also very well know, zionist jews had already gotten out of Germany before the war, those in the camps were mostly the jews who did not want to leave Germany and join the zionist project, they were hated for it by the zionists. Not to mention that not all those in the camps were jews.

And again historically jews, have sacrificed other jews for the cause numerous times, the SS Patria was blown up by the Haganah to gain sympathy (jews killing other jews for the greater cause is common), Yemeni jews were sprayed with DDT on arrival to so called israel, their daughters and young women were prostituted in Tel-Aviv. jews in Argentina brought other jewish girls from Europe convincing them they will get married but later enslaved them for prostitution. You think royalty jews care about some peasant jews who were not onboard the zionist project?

Man you have a lot of digging into, jews rounded up the other jews for the camps, elderly jews were treated as disposable by their brethren. Rich jews refused to pay tiny amounts of money to save their people. There is a LOT of info to dig up!!

Edit: If you want to learn more on how jews tricked and enslaved their own in Argentina look up the book: Sex and Danger in Buenos Aires: Prostitution, Family, and Nation in Argentina (Engendering Latin America)