There is civilian control of the military, and there is unquestioned loyalty to any president. But Trump comes into office with a series of campaign promises that include the possibility of ordering illegal action by the US military.
The issue getting perhaps the most attention is Trump's campaign statements he might institute waterboarding of terrorism suspects. Waterboarding is considered torture, which is illegal under US and international law. Other Trump campaign promises that worry some commanders and troops include the notion of bombing with little regard for civilian casualties or taking Iraq's oil supplies, also potential violations of international law.
Pentagon Press Secretary Peter Cook noted on Thursday that all US troops have an obligation not to follow illegal orders. And he made clear the decades of expectations by troops and commanders that they wouldn't be asked to do so.
"I don't think this is something we should even be discussing," Cook said in response to a question from CNN. "They're not going to get an order in violation of the law."
But CNN has spoken to several current senior officers who have given thought to whether they could confront this issue in a Trump administration.
Each has said the plan will be to explain in detail why it's a violation of the law, and to get military lawyers directly involved in any discussion so that's made clear.
If a commander were still to get an illegal order, the challenge would then be whether to resign.
One senior military official said, though, that the most senior commanders on a four-star level would never resign.
It would be seen as leaving their troops behind to deal with the problem. Instead, the official said, a four-star commander would make a President Trump publicly fire them.
Violations of the Geneva Convention, or any war crime by US troops, would be disastrous, a senior retired US military officer told CNN.
While the US is not a member of the International Criminal Court at The Hague, if there were illegal orders given and followed, then US military personnel would be discredited around the world and US standing would be hurt with other nations the US military must work with.
In addition, those military members engaged in Geneva Convention violations would find they could not travel to countries that are members of the court because they would risk arrest and tried before it.
The official added there is a longstanding view in the US military that American troops and commanders do not engage in activities that are torture or illegal because if US troops are ever taken captive, it gives the the country little leverage to challenge enemy forces engaging in similar activities against US personnel.
Waterboarding, for example, is illegal under both US law and the Geneva Conventions because it is defined as torture.
But that's not how Trump or all the Republicans who endorsed him see it, though.
"Waterboarding isn't torture. We do waterboarding on our own soldiers in the military," Arkansas Republican Sen. Tom Cotton, himself a veteran, told CNN's Wolf Blitzer.
Some US troops undergo training for survival by practicing what would happen to them if they are waterboarded. Cotton also insisted that the law could be changed.
The Pentagon, however, reiterated Thursday that waterboarding is not lawful.
"You know the policy position of this administration, the law in fact, with regard to this. That is not something the US military is going to be conducting," Cook said.