Donald Trump insists an airstrike that killed a top Iranian general was designed to prevent a war with the Middle Eastern country, not to start one.
Iran's promising to seek revenge for a US airstrike that killed a top military general.
America's sending thousands more troops to the region, as tensions rise following Qassem Soleimani's death yesterday.
But several people have criticised the US president, including Senator and Democratic presidential candidate Bernie Sanders.
President Trump defended his actions saying the Iranian regime's aggression in the region, including the use of proxy fighters to destabilise its neighbours, must end and it must end now.
"The future belongs to the people of Iran - those who seek peaceful coexistence and cooperation, not the terrorist warlords who plunder their nation to finance bloodshed abroad.
"The world is a safer place without these monsters.
"America will always pursue the interests of good people, great people, great souls while seeking peace, harmony and friendship with all of the nations of the world."
Mr. Trump's words come hours after the US sent an extra 3,500 troops to the region and, while military sources say this was a long-planned deployment, some of the troops will be based in Iraq.