How does congress use preemption to influence state and local governments?

The concept of preemption shows, through the Supremacy clause, that there are some concepts that are of such national import that the federal government can pass legislation that takes precedence over state and local laws. This could be seen through concepts such as national security and interstate commerce.

They could basically have an unlimited amount, as long as they weren't killed. Like the plantation owners, they had to have incredible amounts of slaves to operate their farms.