Now, the system in the United States is vastly different from that of later generations. Due to its limited involvement in World War I, the United States had not yet abolished the segregation laws between the states.
In layman's terms, the U.S. army had no power to enter any state without the state's permission, otherwise the states had the right to consider the actions of the U.S. army as an invasion.
All states also had their own troops, and the strength of the National Guard was not necessarily weaker than that of the U.S. army.
Especially in some of the more powerful states, if a governor took a dislike to the President, they could even put the President on the state's blacklist and forbid the President from entering the state's land.
Yes, it sounds that incredible.
Although the federal system allowed America to quickly acquire a large amount of territory and it took just over 100 years to connect the East and West Coasts of America.