* Patently illegal conduct, according to common person principle, is always illegal. There are no legal exceptions.
* If you are fired upon you must return fire. Uniformed militaries are obligated to defend themselves. There are no exceptions to this, except the prior point. When these two points are in conflict the prior point always wins.
* Uniformed service members are required, by law, to follow orders given to them except for the prior two points.
That is the law. It does not matter what specific scenario finds tenable or practical, because combat is inherently challenging. In most cases this is highly impractical, which is why urban warfare is so challenging.