Why is federalism important to the study of American history?

To distinguish it from nationalism.

The US was a federal union, not national: i.e. an international confederation of separate sovereign nations like the UN or the EU.

In 1783, the individual states were supremely ruled by their respective state governments under the Articles of Confederation.

Then under the Constitution, every state was supremely ruled by its respective voters-- not any government officials, who were their mere subordinates, and the state voters could overrule them at will.

Then usurper-presidents like Jackson and Lincoln, re-wrote history; to falsely claim that:

• the states formed a single nation in 1776,
• declaring their sovereign dependence on each other as a single nation of 13 dependent states.

This created a nationalist policy, in which the federal government officials had absolute power; while the people of each state did not consent to their government.