Although uk / usa would love to smash iran & over power them for oil. ( war criminal cock roach blairs mantra for years ) If you watch " george galloway on iran on RT " iran is the exact opposite to 2003s iraq . In every way.
Millions of possible people who would enjoy fighting the west.
The s *** storm it would produce would be mega.
True, the mess that was Vietnam would pale into insignificance compared to taking on Iran, even the Saudis have backed out (and probably told Trump to).