Technically only Congress can authorize a war. However, the president can and often will undertake “peacekeeping efforts” or “counterinsurgency operations” or “targeted strikes” without congressional approval.
Whether anyone could stop a president issuing an order is another question. The president is the commander in chief - the military reports to the president, not Congress. If the president tried to order the military to do something unconstitutional (like fight a war that was not authorized by Congress or, idk, overturn an election) then we’d be in a constitutional crisis. In such a crisis, either the military disobeys the president (which is unconstitutional) or the president violates separation of powers (which is unconstitutional)
The American system of government relies on three branches all participating in good faith. As soon as that stops, it all falls apart. Though government is just a series of rules and norms. Rules and norms won’t stop soldiers all the time.