Presidents fail when their policy objectives become so important to them that they take over their administrations and lead them to destroy themselves.
Johnson in Vietnam, Biden in Afghanistan, and George W. Bush in Iraq are potent examples of how idealism gone astray took over a presidency and led it to ruin.
Will Iran serve as just such a snare for Trump and will his pursuit of regime change lead him to self-destruction?
In Vietnam, in Afghanistan and in Iraq, Presidents Johnson, Biden, and Bush-43, let military domination blind them to their adversaries' ability to survive massive battlefield defeats and come back for more.
In each case, the willingness of our adversaries to accept casualties, economic and human, exceeded the ability of the president and the American people to endure pain, leading us to humiliating defeats.
We always fail to realize that a massive military arsenal is no match for our enemy's fanaticism.
It's easy to see how military superiority could lead Trump to over-confidence and his administration to ruin.
The solution is for Trump to abandon regime change as his goal and instead focus on denying Iran the ability to spread terror globally.
Don't predicate our victory or defeat on our ability to make Tehran see reason and to act based on enlightened self-interest. We must not let our easy win in Venezuela lead us to believe that we can persuade the zealots in Iran to act rationally.
We likely cannot teach Iran to respect the human rights of its own people.
But we can make it impossible for the Iranian regime to spread terror throughout the world and deny it access to nuclear weapons.
That more modest aim, still very worthwhile and deserving, must become our objective even if Iran is allowed to perpetrate horrible harm on its own people and to refuse regime change.
By scaling back our war aims, we can still succeed at the key task — curbing Iran's ability to inflict massive global economic harm — and emerge "victorious" in this war with our president and our party intact.
We can't stop Iran from destroying the global economy, but we can stop it from humiliating us.
And that will be good enough.
Dick Morris is a political strategist and author who has advised several U.S. presidents, governors, and mayors over a 40-year career. Read Dick Morris' Reports — here.