I think something bad needs to happen to the US on the home front. Besides being one of the biggest powers in the World Wars, none of the fighting took place in the 48 States. This meant that we got the benefits of war (e.g. increased industrialization) without the biggest devastators (e.g. trenches snaking across farmland which is muddy all the time and fallow because it's too dangerous to plant anything and if you did it would get trampled anyway).
Unfortunately for the challenge, however, there's nobody next to the US able to challenge it. Some sort of civil war is probably needed. Or perhaps a British invasion from Canada, although the US is still very big.
Unfortunately for the challenge, however, there's nobody next to the US able to challenge it. Some sort of civil war is probably needed. Or perhaps a British invasion from Canada, although the US is still very big.