The US will win, no doubt about that. But this war will also push the US back by a decade in terms of population, while also screwing up oil prices for everyone else.
Iran is not Iraq or Afghanistan. There's no comparison at all. And if the US is going to struggle a lot against a country like Iran, then what do you think will be your expectation from India if Pakistan is considerably stronger than Iran today, and a nuclear power no less?
I dunno why you brought that up, but WW2 was a very different time. Anyway, if your question wasn't rhetorical, then the Germans had 4.2 million and the French had 5 million.
In comparison, against Iraq, both sides had a little over half a million combined during the major war, with NATO being in greater strength. And NATO also had support from 300,000 armed Kurds. After the war, NATO had deployed 180,000 soldiers and used more than 500,000 Iraqis themselves as an occupation force, alongside the Kurds. So well over a million to occupy Iraq.
Otoh, Iran can raise more than a million troops for the major war alone, and an unknown number after NATO has occupied Iran. So you can expect NATO to be fighting with lesser numbers, which is a very bad thing considering how badly they have fared even with a numerical advantage. And you can bet the Iranians will have far more motivation to fight than the Iraqis did. They also have this militia called Basij under the IRGC and claim to have a ridiculous 11 million men and women in it that are trained to fight, and they claim 600,000 are immediately available for mobilisation.
And to make matters worse, unlike Iraq, Iran is modernising. With a lot of Russian help, they have reverse engineered a lot of stuff, including battle tanks, ATGMs, long range SAMs etc. They even plan to license produce the Su-30SM after 2020, when sanctions are lifted.