The Trump administration committed itself last week to an indefinite military deployment in Syria despite having no apparent legal authorization to do so. We're all for staying on the offensive against terrorist groups like Islamic State and al-Qaida, but there are proper steps for carrying out such missions. President Donald Trump must follow the law, not personal whim.

President Barack Obama handled the 2011 start of Syria's civil war clumsily. A window of opportunity briefly opened to end a murderous dictatorship and plant the seeds for democratic governance. As Obama fretted over a military commitment, moderate rebel groups faced an onslaught by Russian- and Iranian-backed government troops while radicals, eventually including Islamic State, swept across the countryside.

President Bashar Assad correctly sensed U.S. ambivalence and felt emboldened when Obama drew his infamous red line over chemical weapons, then failed to intervene forcefully when Assad used them against his own people. Those attacks are still occurring, as top U.S. officials confirmed on Tuesday.

Neither Trump's military bluster nor the presence of an estimated 2,000 U.S. troops in Syria has deterred Assad any more than did Obama's approach. So why keep a presence there?

Secretary of State Rex Tillerson stated last week that troops would remain indefinitely to ensure Islamic State fighters didn't regain a foothold. Such a policy was understandable when Assad's government was collapsing and Islamic State was scoring massive territorial gains. But Islamic State no longer poses a threat either in Iraq or Syria. Assad is back in control over a sovereign Syrian state.

The U.S.-led wars in Afghanistan and Iraq occurred with U.N. authorization as required under international law. When Iraq's sovereignty was restored in June 2004, any ongoing U.S. military presence there could only occur with Iraqi permission.

Syria never gave such permission. In fact, it specifically protested the U.S. military intervention. Under international law, the U.S. troop presence is unauthorized. Until now, Assad has simply tolerated it. Russia, by contrast, has a longstanding agreement with Syria to base troops there. Iranian forces and Lebanon's Hezbollah militia also are there by invitation.

So far, these disparate forces have managed not to clash. But the threat is escalating. NATO ally Turkey is angered by U.S. support for Kurdish fighters in northern Syria. Turkey recently launched a cross-border offensive and is threatening an attack on positions held by U.S. troops and Kurdish fighters in the border city of Manbij.

This is not a fight America needs or can legally justify. If Trump feels differently, he should state his case publicly and seek congressional approval. Having pledged as a candidate in April 2016 that, under his watch, America would "not go abroad in search of enemies," he's now in the process of doing exactly that.

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