Why did so many military-aged men immigrate to the US during an active civil war in which they were subject to military draft? Why did they believe in the cause of the Union so fervently that they were willing to sail all the way from Europe to die in battle for those values?

Newt Gingrich is right - mass-immigration is the only reason that the good guys beat the bad guys in the US civil war:

https://www.twitter.com/newtgingrich/status/1958827407940436058

If you oppose immigration, you are fundamentally anti-American.