The Pentagon is conducting a major review as Iran announces the U.S. military has been added to a list of groups threatening regional stability.
Turkey and Iran, traditional foes, have established common ground in Iraq, Syria and Qatar as regional alliances shift.
Military leaders said they did not fear an invasion from the West, touting Tehran's growing military might at home and abroad.
"The Americans will take this dream to their graves," a senior adviser to Ayatollah Ali Khamenei said,
Former U.S. and Israeli generals disavow a possible U.S. military strike on Iran, and a European scholar says Trump may find himself isolated.
Majority-Shiite Muslim neighbors Iran and Iraq have a volatile history and differing views on the U.S., but they share an interest in battling jihadis.
The three countries all want a stake in Syria's future and appear willing to secure it by making long-term military commitments.
More than 64 years after the CIA helped overthrow Iran’s democratically elected government, the country reacted to recent U.S. comments on regime change.
The U.S. chopper reportedly triggered its automatic defense system, setting off flares.