Because the War Powers Act has been enforced for over forty years and it has not been adjudicated as unconstitutional yet, there is a strong incentive to consider it constitutional. However, it has not been challenged in court yet, so it is possible for it to be declared unconstitutional at some point.
The default process to begin an armed attack is for it to be approved by Congress. If the president does not do so, the burden of proof lies with him.
Congress appears to be in the right, as they ordinarily have to approve any armed attack. Moreover, the president not only avoided consulting with Congress, but he did not provide a report in a timely manner.
The most important thing to find out is why the President believed that this use of force did not require approval from Congress. In order to establish that, we need to ask whether the attack to civilians was indeed imminent, whether the deployment was so small as to not warrant a declaration of war and whether the president had any other reasons to not consult with Congress.
As for Congress, we would need to ask whether they believe this act needed their approval, and the reasons for this belief. We would also need to ask them whether they believe it is a good idea to remove the troops at this point, and the consequences this could have.