They were there under the UN banner alone with observers from a dozen other nations and they were unarmed, which means they do fuck all.There were US soldiers in Georgia during the 2008 South Ossetia war too, and look how that worked out...
Military tourists, except their postcards are 500 page reports that get filed and forgotten once they return from their trip.
Russia prevented them from returning after their mission expired using their Security Council powers.
The Russkies aren't stupid. They won't fire on a bunch of unarmed UN observers.
They just wait until the mission expires and the use their special UN powers to stop them from returning.
US forces in Ukraine are armed and under the US flag, just like in Syria.Training detachments and such generally get out of the way when shit hits the fan, as far as i know this isn't a tripwire force, like in certain NATO countries or in South Korea.
Russians didn't mess with US forces in Syria and they won't in Ukraine.