A boots on the ground invasion would cripple Iran
I am not even sure how they would do that so easily. With the previous Gulf Wars, they have come through Saudi Arabia.
Iran's geography makes it much harder.
worldview.stratfor.com/article/geopolitics-iran-holding-center-mountain-fortress
Under the current circumstances, it might be useful to benchmark Iran against Iraq or Afghanistan. Iraq is 433,000 square kilometers, with about 25 million people, so Iran is roughly four times as large and three times as populous. Afghanistan is about 652,000 square kilometers, with a population of about 30 million. One way to look at it is that Iran is 68 percent larger than Iraq and Afghanistan combined, with 40 percent more population.
More important are its topographical barriers. Iran is defined, above all, by its mountains, which form its frontiers, enfold its cities and describe its historical heartland. To understand Iran, you must understand not only how large it is but also how mountainous it is.
And we all know how the US have fared in Afghanistan and Iraq.