The evidence exists if you care to look. Most of it is in the way of first-hand accounts, however. Seeing as how this particular forum lacks a subpoena power over defected Shi'ite militia members, I have no way of directly presenting you the evidence.
Indirectly, however, you'll find the Internet is pretty replete. You can start here: https://en.wikipedia.org/wiki/Iran_and_state-sponsored_terro...
Given the strident tone of your post, I suspect you'll dismiss those sources. No doubt you'll suspect them of being self-serving. Very well. Evidence of Iran's doings also exists in forms which cut against the U.S. military's interest, such as the fact that they duped the U.S. military into violating U.S. law by providing them weapons.
https://www.jpost.com/Middle-East/Is-the-US-supporting-Irani...
https://www.alaraby.co.uk/english/indepth/2017/9/4/how-iraqi...
https://abcnews.go.com/International/dirty-brigades-us-train...
The fact is that Iranian-sponsored terrorism is so prevalent in the Middle East, particularly in Iraq, that just defeating Daesh is a moral problem... it's become a logistical nightmare to not only perform Leahy vetting but to actually carry out operations without a Shi'ite militia taking part and committing atrocities in the name of the Coalition.
And those Shi'ite militias? "All are directly backed by Iran’s Revolutionary Guards Corps and are notorious among the Sunni minority population for carrying out extrajudicial killings, torture and ethnic cleansing."
https://www.thedailybeast.com/iraqs-shia-militias-accused-of...
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You are entitled to your opinion that the "USA is just a big bully," although I personally don't think it's a well-founded opinion. However, the next time you feel inclined to start calling things a "farce" before you've examined any evidence, I'd encourage you to reconsider.