I've thought this through.
The military is both loyal to the Constitution and wary of the idea of fighting an insurrection at home - the generals know exactly how bad it gets because they have just been doing that overseas. They have every reason to do what it takes to put back the country together with minimal harm, given a crisis where they are called upon to make a significant commitment of forces. But they won't act until they start getting orders that will trigger such a crisis - i.e. to do unconstitutional acts at home with the goal of putting down insurrection.
And that presents the worst case scenario: Months to years of military governance that is premised in reestablishing the values and principles of the USA. This will necessarily bring out some related questions around the founding document and overturn some historical precedents.
But it will not turn into a dictatorship because that makes the loyalties compete with each other: You would need a leader so strong, so approved of as an ideal, that they can overturn centuries of belief in the nation. Unlike countries that have little history in this regard and are prone to instability, the only way any such leader can look legitimate and maintain power is by following through on the core values of the USA and therefore winning "hearts and minds". As such they would reinstall a republic in due time.
And that is the worst case, as I see it. It only gets milder from there, with more of the reforms coming from within the existing system.
