The Allies choose to wait, defend, build up. India is lured with freedom to send its armies into Japan. This distracts the Japanese, and the Americans successfully drive them off American territory and attack the Solomon Islands. Still, they are a long way from defeated.
The British, meanwhile, use their navy to inhibit the Germans, who are trying to occupy French colonies. With British help, the German supply lines are cut and the colonies remain untouched ... yet.
However, the USSR is still free, and it attacks the Northern US on the East coast, unexpectedly, force their way through the defenses to capture Boston. The Americans in the area form a defensive line around the Soviets but are unable to recapture Boston. To prevent the Soviets from gaining an advantage, the US president orders the usage of Operation Manhattan. Three days later, a massive explosion destroys an an entire division of Soviets coming to re-inforce Boston. The city, without supplies or re-inforcements, soon falls. A second weapon is used on Berlin. The city, along with the Greater Germany government, is extinguished in a single nuclear flame, and Germany falls into Civil war. Meanwhile, the UK starts building more of said weapons, and attacks France ferociously, then Spain, then Switzerland, liberating them as it goes along.
But a new government has risen in Germany, and it commits its armies forward once again. Britain can hold them, but they need every soldier. Their spies, however, have revealed a USSR strike into America. The Americans are stretched. They cannot defend against this. But if the British leave Europe to defend America, Germany will once again control Europe.