Members of the female division of the People's Protection Units (YPG), carry the body of comrade Roj Adiyaman, one of eight soldiers that were buried in the martyrs' cemetery on November 8, 2015 in Derek, Rojava, Syria. The U.S., Russia and Iran are all involved in the Syrian Civil War. They are all working to defeat ISIS, but for different reasons.

The president of the United States has very different attitudes toward the two biggest supporters of one of America's greatest enemies.

Both Iran and Russia support Syrian strongman Bashar Assad, who has committed atrocities against his own people in that country's bloody civil war. Trump has been candid about his distrust of Tehran, but he has been less critical — sometimes even supportive — of Moscow.

Trump is a recurring and vocal critic of Iran. He opposes a nuclear deal struck in 2015 between Iran and a group of major powers including the Obama administration, calling it the "worst deal ever" and vowing to "rip it up."

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In a May speech in Saudi Arabia, Trump condemned Iran for supporting militias, terrorists and other extremist groups.

"Among Iran's most tragic and destabilizing interventions have been in Syria. Bolstered by Iran, Assad has committed unspeakable crimes," he said.

With Russia, Trump has taken a more positive tone. He has tweeted that having a good relationship with Russia is a good thing, and that only "stupid people" would disagree.

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The U.S. has been slow to realize Russia's "power projection issues" in the Middle East, said Andrew Bishop, deputy director of research at the Eurasia Group, whereas Iran ambitions in the region are longstanding and well-known. Plus, Bishop said, Trump may perceive Russia as a fellow target of Islamic terrorism and one of few potential partners in Syria.

The president has voiced his desire to work with Russia to fight the Islamic State group, also known as ISIS.

The U.S. Senate appears ready to ensure that the administration doesn't unilaterally ease up on Russia. On Wednesday, the Senate voted overwhelmingly to require the president to get congressional approval before he lifts any sanctions against the country.