The last president who actually declared war against an actual enemy was Franklin D. Roosevelt. From Ron Paul at ronpaulinstitute.org:
Last Thursday President Biden continued what has sadly become a Washington tradition: bombing Syria. The President ordered a military strike near the Iraqi-Syrian border that killed at least 22 people. The Administration claims it struck an “Iranian-backed” militia in retaliation for recent rocket attacks on US installations in Iraq.
As with Presidents Obama and Trump before him, however, Biden’s justification for the US strike and its targets is not credible. And his claim that the US attack would result in a “de-escalation” in the region is laughable. You cannot bomb your way toward de-escalation.
Biden thus joins a shameful club of US leaders whose interventions in the Middle East, and Syria specifically, have achieved nothing in the US interest but have contributed to the deaths of many thousands of civilians.
President Trump attacked Syria in 2018 in what he claimed was retaliation for the Assad government’s use of chemical weapons against its own citizens. The Trump Administration never proved its claim. Logic itself suggests how ridiculous it would have been for the Syrian president to have used chemical weapons in that situation, where they achieved no military purpose and would almost certainly guarantee further outside attacks against his government.