What does Hitler mean when he writes “an ancient instinct admonishes all peoples to set aside internal controversies when the fatherland stands at the decisive hour for political action.”?

Hitler here is calling for the unification of the Aryan people.

Explanation:

Hitler here invokes the ancient history of the Aryans that allegedly traced them back as the original master race that is to rule the whole world.

This is the idea that he is hankering back to in here as he claims that the people must go back to the ancient instinct they can find themselves within and following that same instinct they must unite against the enemy as the Aryan people

The connect of the Aryan race was in itself a careful calibration at the part of the party and Hitler but it was the main tool he used.