At what point do you want that justification to be made? When you are at the pointy end of the spear you can't presume to know enough to make that call. You have your mission and objective beyond which you have limited information.
Now, you do have responsibilities within that mission that fall within the Laws of Land Warfare and are expected to uphold those responsibilities even if the situation on the ground is different than what you were told it would be. For example, if you are told the objective has no non-combatants but find women and children on site, then you have to stop and take steps to protect them while you execute your mission.
Are there times when military officers should question? Absolutely. For example, during the LA Riots active duty units were sent out to help the situation and there were huge questions from all along the chain of command as to whether it was legal and how to handle things on the ground. There are lawyers and commanders from Brigade, Wing and Task Force level on up looking at these things so the joes can ruck up and execute the mission.
The next issue I see is that words and actions matter. In other words, a declaration of war means a lot more than a short term armed incursion. Look at WWI especially. The treaties were written to lock in actions that would follow the declaration of war. If Germany declared war on France, then Britain had to declare war on Germany, etc, etc.
When we found Cuban military in Grenada we could have declared war on Cuba. But we didn't because we thought that might have resulted in armed conflict with the Soviets. Instead we chose to protect American lives with a short term armed incursion that got rid of the Cubans and reinforced our interests in the region. In my experience, I would rather take that approach than risk the larger ramifications that come with a declaration of war. And I'm okay with letting the powers above me decide if it is constitutional or not.
