....however- on federal or state government buildings it should not be tolerated because a treasonous act was waged against the union. THAT would be 'political correctness' run amok.
A point of debate which took an invasion of the North against the South to resolve.
It is not an invasion when an army maneuvers and travels within its own territory. The country was the United States, and the U.S. Army moved within the United States to deal with a rebellion.
One of the first things I learned in studying history was not to apply 20th Century ideas and mores to 18th or 19th Century people.
You are smart enough to know that history books are written by the winners. Your attitude on the Civil War is dictated by a post-Civil War perception. If you read about it from a pre-Civil War perspective, then it becomes a lot clearer that "one nation" doesn't exist as we think today. People put their states first and that small Federal government in Washington, D.C. was a distant entity which dealt with England and the Barbary pirates with little to no effect on the general population.
Noted Civil War historian Shelby Foote once put it this way:
Before the war, it was said “the United States are.” Grammatically, it was spoken that way and thought of as a collection of independent states. And after the war, it was always “the United States is,” as we say to day without being self-conscious at all. And that’s sums up what the war accomplished. It made us an “is.”
As Foote noted, the Civil War changed how we thought of states and the Federal US. That was post-war, not pre-war. Robert E. Lee, a West Point graduate and experienced US Army officer, didn't agree with the idea of secession, but when it happened, felt compelled to defend his "country". His "country" being Virginia because that how Americans thought; their state was their country. Some of the older farts like myself on this forum remember as late as the 1960s and 1970s that people often put their state ahead of the Federal government.