Donald Trump, in his first major usage of the commander in chief title, has ordered a strike on a Syrian military target. Since he ordered it without the authorization of Congress, however, the question of whether Trump's Syria missile strike was constitutional is still very much up for debate. The law regarding war powers would suggest that the president cannot order a military action against another country without the support of Congress — but it has been interpreted differently over the years.
Taken alone, the Constitution says that Congress must declare war against a foreign power, and then the president as the commander in chief directs operations once war has been declared. After the Vietnam War, where three presidents had sent troops into battle without the authorization of Congress, the legislative branch stepped in with the War Powers Resolution of 1973. This enforced the strict separation of powers, stating that the decision to enter the armed forces into hostilities had to be justified either by an act of Congress or a national emergency resulting from an attack on the United States.
However, American presidents from both parties have seemed to agree that this law itself is unconstitutional, limiting their rights as the leader of the military. Many have acted in ways that at least some members of Congress have deemed illegal — and this latest strike is no different.
Congress seems to be pretty split about this strike, and not directly along party lines. The more libertarian leaning members of the Republican Party, including Kentucky Sen. Rand Paul and Michigan Rep. Justin Amash, have come out vocally against the strike. They have been joined by some high-profile Democrats, including Sen. Tim Kaine and Sen. Dick Durbin, whose response did not explicitly condemn the attack but did urge the president to consult Congress before making any other moves.
The president has found plenty of support in Congress, though, mainly but not exclusively from conservatives. Paul Ryan, John McCain, Lindsey Graham, and Marco Rubio were among those saying that this was an appropriate response to Syrian President Assad's alleged use of chemical weapons (his regime denies involvement). While Obama's decision not to act on Assad's atrocities against his own people is something that many criticize Obama for, Trump's opposite reaction to another chemical attack is still not necessarily the right decision.
The bottom line is that those calling the strike unconstitutional do have legitimate standing to be saying so — but on the other hand, there is also plenty of historical precedent for presidents to order military actions without congressional approval. This could be the beginning of a long engagement in Syria, so hopefully the president and Congress will have plenty of time to work out an agreement between themselves.