Iran is trying to gain a military base near the Israeli-Syrian border, a bipartisan pair of lawmakers warned the Trump administration.

"A permanent Iranian military base in Syria, potentially near the border with Israel or Jordan, would increase Iran's operational capacity to inflict serious damage against two of our closest allies in the region," Rep. Peter Roskam, R-Ill., and Rep. Ted Deutch, D-Fla., wrote in a letter to Secretary of State Rex Tillerson.

That warning points to the strategic significance of the Syrian civil war, beyond the importance of defeating the Islamic State. Trump's team has authorized an intense barrage of airstrikes on ISIS, while opening the door to cooperation with Russia; a senior State Department official recently attended a peace talks summit between Syrian President Bashar Assad and opposition forces that was led by the Russians. But Russia is propping up Assad in partnership with Iran, which means the defeat of ISIS could inaugurate a new phase of regional rivalry.

"The day after Raqqa [the capital city of ISIS in Syria] falls is going to be the moment that Iran moves to try to oust the United States from the region," Sen. Marco Rubio, R-Fla., told the Washington Examiner in February.

Roskam and Deutch have a similar concern. "Iran is actively working to establish a permanent military presence in Syria," they wrote Tillerson. "The Islamic Republic seeks to solidify its access to the Mediterranean Sea by building a permanent seaport and constructing numerous military installations throughout the country. A permanent Iranian military presence in Syria would greatly harm U.S. interests in the region and decrease the likelihood of reaching a political agreement to end the Syrian civil war."

The lawmakers want Tillerson to try to weaken Iran by cracking down on the regime's commercial aviation industry, which is often used to send weapons to Assad, and by imposing sanctions on Iran that would hurt the regime without violating the nuclear agreement that the Obama administration negotiated. "It is imperative to develop and implement policies to prevent a permanent Iranian military presence in Syria," they wrote.