U.S. intelligence showed that Iran has made plans to target U.S. forces in Iraq and elsewhere in the Middle East, triggering a decision to reinforce the American military presence in the region in an effort to deter any possible moves by Tehran, U.S. officials said Monday.

The escalation in tensions came as European diplomats said Monday that Iran appeared poised to breach portions of the 2015 international nuclear pact that restricted Tehran’s nuclear program in exchange for relief from economic sanctions. That followed a...