President Donald Trump‘s decision to align with Israel in targeting Iran’s regime, including strikes that U.S. and Israeli officials say killed Supreme Leader Ayatollah Ali Khamenei and other senior officials, has exposed tensions inside his “Make America Great Again” coalition that propelled him back into office.

Trump signaled Monday that the U.S. military campaign against Iran could stretch “four to five weeks,” warning that more U.S. casualties may be ahead as the conflict deepens, remarks that are testing the limits of the “America First” movement that helped power his presidency.