The Orthodox were persecuted and probably by some of the same folks.
Try having all your Churches seized by a foreign king. and being arrested or killed if you didn't add 'and from the son' to your recitation of the creed.
Still the Ukrainians got off easy, at least they could still write in their language, unlike the Belarussians when they underwent similar persecution.
Frankly lookup Costantinople in 1250, and Zara some months earlier. Pretty much the first Christian on Christian persecution in history, and the Orthodox were on the receiving end.
To say nothing of what the athiests did, sending more Orthodox to death in workcamps than the Nazis did Jews.
I wouldn't say it was the first Christian on Christian persecution. When the State got involved in Rome, there were several groups that would have had the basics right that were nevertheless persecuted for this that or the other.