> since when are American army resources used to protect European borders and shipping routes?
Since the 1940s.
> And from who?
Since WW2: Russia. The US military's position in Europe post WW2 enabled the destroyed, fragile nations of Europe to rebuild in the face of a Soviet onslaught to conquer as much territory in Europe as they could.
Also: Serbia, right in Europe's backyard. See: the Kosovo War. [1]
Also: Ukraine today. The sole reason Ukraine exists at all still, is that the US poses a severe threat to Russia were it to attempt a full annexation move using its military. They couldn't be certain how the US might react. Remove the US from that equation, and Russia knows the Western European powers will do nothing to stop it from taking Ukraine (other than protesting at the UN and throwing on some sanctions).
Russia would immediately begin annexing territory in several prominent Eastern European nations, along with militarily challenging several others (eg Poland), were the US to pull its military resources and declare it had no interest in European affairs. The Western European powers would do nothing to stop that and could do nothing. France and Britain have nuclear weapons, which would only be used as a threat if their own territory was at risk (which it would not be; France isn't going to nuclear war with Russia over annexed Eastern European territory). Among Western European nations only Britain and France are capable of operating their militaries beyond their own borders properly, and in those cases they can barely do that. This is fundamentally why it was the US that had to intervene to stop the Muslim genocide in Kosovo, because the Western European powers can't project their militaries well and lack the capabilities the US has.
[1] https://www.hrw.org/report/2001/10/27/under-orders/war-crime...