How do governments get their just or rightful powers in the declaration of independence?

Answer: Governments get their just powers from the consent of the governed" - The powers of government to secure the Creator-given rights must be just, and they come from the consent of the people.

Explanation: The government got their power from the people,

That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on.

Unfortunately, whenever a revolution occurs warfare always follows. This can be seen in both the American and French revolutions. And more recently with the first Sudanese civil war.

tl; dr

More often than not governments get their rightful power through war