Are you alluding to Allies vs Axis? Because I don't think US have invaded a country unilaterally in that war. It would be similar if Mexico/Canada/Cuba togethe

Are you alluding to Allies vs Axis? Because I don't think US have invaded a country unilaterally in that war. It would be similar if Mexico/Canada/Cuba together declared war at US and then Russia joined them. Because then sure Russia can have a decent chance of successfully invading US across a sea and then strong military could make a difference. But Mexico and Canada won't and countries who could are inconveniently far. This is why I say lack of attempted invasions may be more thanks to good geopolitical situation and civilian population equipped for defense than strong military.