Why do states have to follow certain federal rules?

The supremacy clause contains what's known as the doctrine of pre-emption, which says that the federal government wins in the case of conflicting legislation. Basically, if a federal and state law contradict, then when you're in the state you can follow the state law, but the feds can decide to stop you.

Explanation:

of what agreement it does not show

the civil war started because of uncompromising differences between the free and slave states over the power of the national government to prohibit slavery in the territories that had not yet become states. when abraham lincoln won election in 1860 as the first republican president on a platform pledging to keep slavery out of the territories, seven slave states in the deep south seceded and formed a new nation, the confederate states of america. the incoming lincoln administration and most of the northern people refused to recognize the legitimacy of secession. they feared that it would discredit democracy and create a fatal precedent that would eventually fragment the no-longer united states into several small, squabbling countries.

i believe it caused a regional impact in societ