> This is why it was necessary to beat the Germans first - so the Russians could invade or at least threaten Japan.
That was the strategy that FDR and Churchill agreed to, but the primary person driving it was Stalin, because he didn't want the US and Britain to defeat Japan before he got a chance to attack them, and he knew he would not be able to attack them until Germany was defeated. (He wanted to attack them so he would have a pretext for taking over territory that Russia had lost in the Russo-Japanese war in 1904-1905.) As events proved, the US and Britain did not need any help from the USSR to defeat Japan militarily, and with different diplomatic choices they probably could have gotten Japan to surrender before the USSR attacked.
Once FDR died and Truman took office, btw, it was no longer clear that having the USSR enter the war against Japan was a US objective. Truman, unlike FDR, was not a fan of Stalin and viewed him as a geopolitical threat, not an ally. Which, historically speaking, was a sounder view.
That was the strategy that FDR and Churchill agreed to, but the primary person driving it was Stalin, because he didn't want the US and Britain to defeat Japan before he got a chance to attack them, and he knew he would not be able to attack them until Germany was defeated. (He wanted to attack them so he would have a pretext for taking over territory that Russia had lost in the Russo-Japanese war in 1904-1905.) As events proved, the US and Britain did not need any help from the USSR to defeat Japan militarily, and with different diplomatic choices they probably could have gotten Japan to surrender before the USSR attacked.
Once FDR died and Truman took office, btw, it was no longer clear that having the USSR enter the war against Japan was a US objective. Truman, unlike FDR, was not a fan of Stalin and viewed him as a geopolitical threat, not an ally. Which, historically speaking, was a sounder view.