The theory is - assassinate Iran's top military leader and you weaken Iran's ability to wage war outside its borders. True, General Soleimani was vital to Iran's military, and his absence will, at least temporarily, reduce Iran's ability to support groups outside of Iran that the US has deemed to be terrorist organizations, at least until someone equally able takes his place.

However, this General was loved by the Iranian people. He was fiercely nationalistic and always put the country and people of Iran's welfare at the front of his mind. No country wants an outsider to come in and kill off one of its leaders. Already demonstrations are raging against the US as the death chants to America intensify. And also now the Islamic Republic regime is able to rally the people behind a common theme of the common enemy, which always works (ask Trump!) to rally support.

Which means, that at least in the short run, Iran's government now has the fodder it needs to maintain and justify its anti-US stance, as well as to rationalize its very existence, in a war time struggle with an outsider.