At some point a military careerist realizes that the important task is to provide your political sponsor the justification for the predetermined course of action. While a warrior might be focused on the defense of the nation, the Perfumed Prince of the Pentagon understands that the future is paved with rationale for where the boss already has decided to go.
If you know that the President is totally averse to engagement, you create a scenario of impossibility to justify that course of (in)action. Prove that it is simply too tough to do.
But, if you understand that the need is to preserve the existence of your nation, then you weigh the alternatives, evaluate the risks, plan the mission and tell the leadership what the real options are. The game may well be worth the candle.
A Range of Options In Two Nations
That is probably the clearest comparison you will see. In one example it is proven impossible. In the other, it was simply done.