hmm, I dont entirely disagree, it depends on the country, and strategy.
England, Germany and France probably wouldnt work out well for them, Italy would die out in a day, Japan only has a meagre "self defense force" Israel has some of the best soldiers in the world and decent armour, but there are simply too few of them. China's army is vast, well supplied, very disciplined and have a huge population base to draw conscripts from they would overwhelm the states on numbers alone. Russia, who knows, maybe back in the soviet regime, but probably not now. Canada has an elite air force, and our soldiers are well trained, equipped and supplied, but training takes too long there simply arent enough of them. furthermore our navy is total ass, a few second hand submarines, a couple decent destroyers and a frigate. plus about 100 inactive warships. No aircraft carriers either. The best strategy to use against us would be to cut off our escape and shower us with bullets, China could be beaten if you dig in and have good air support. Italy you just have to blow on. England should be cut off from the outside world and methodically exterminated, without resources or the ability to import they would not be able to recover losses of materiel, Germany cant be beaten by naval means, their army is the best asset they have, you would be hard pressed to best them on land, some of the best tanks out there now are german, as a result the air force an navy wouldnt much come in handy, as the german air force can hold its own as well, plus what navy they do have is top of the line, dont go picking a fight with them. France has a surrenderng habit, beat them into submission until they surrender, expect the war to last a week, tops. for Israel, , same as canada, trap em and keep killing them till theres none left. Russia is a big place, they will be a challenge even without a considerable military, because if they can mess around with reinforcements and supplies, you will be too spread thin to mount an effective campaign, keep it together, dont make the same mistakes the germans made in WW2.
Happy warmongering, and good luck.