Earth vs the Moon, Mars and maybe an atmospheric Venus base, triggered over exploitation of lunar resources by multi-planetary corporations, with insufficient compensation, lack of political representation and poor living conditions resulting in an interplanetary terrorist attack that triggers an overwhelming backlash.
I'll take a potshot and predict it in the 23rd century.
Just a hunch, but it seems to me that in an event to be called WW3 there would be a lot of infighting. There just appears to be a lot more civil unrest in recent decades, and I just have a feeling it would be a war much more fragmented than the ones before it.
I don't think China blowing up a satellite would be enough casus belli to bring the US into a war. No loss of life would occur.
Going back to World War 2 as an example, where the US similarly provided munitions aid to Britain via lend-lease, Germany sank several US aid ships, killing their American crews, and that wasn't enough to bring the US into the war in full until the bombing of Pearl Harbor.
It would need to be some sort of event that brings conflict to the US, like Pearl Harbor or 9/11. Americans are too war-averse otherwise to support direct involvement in conflict, preferring proxy wars when they see the need for intervention.