Who invaded who?"
See? That's your problem -- you only know those pieces of history you wish to know and ignore or just deny the rest.
In fact, Bull Run was far from the first battle, or "invasion", it was the eleventh.
Before Bull Run there were ten other battles producing a total of over 1,000 casualties, but you don't know about them because you don't wish to know.
Of those ten prior battles, one was the Confederate assault on Union troops in Fort Sumter, two were in Missouri, three in West Virginia and four in Virginia.
Of those 10 battles, Confederates won four and suffered about 700 casualties, the Union won six and suffered 360 casualties.
In all, 1861 saw 35 named battles, 25 in Union states & territories, 10 in Confederate states.
Those 35 battles produced over 15,500 casualties of which Bull Run represented about 30%.
Winners & losers: in 1861 the Union won 11 battles in the Union and 3 in Confederate states, Confederates won 10 battles in the Union and 5 in Confederate states.
Six of those 35 battles in 1861 are classified as "inconclusive".
Bottom line: 1861 was not "all about" Bull Run, rather it was "all about" the Confederate invasion of the United States, where 25 of 35 battles were fought and where Confederates suffered 1,000 more casualties than they did in Confederate states, even including Bull Run.