Why not? Worked on da Jooz.
Actually, it didn't, or the Nazis would have reduced their numbers even more. Looking at how they handled things is a learning tool for dealing with large numbers. With the exception of the extermination camps.
WRT control, it does involve mobilizing the army. You need a large number of armed individuals who can effectively corral the large numbers of people. You need an org that has the equipment, and know how to gather up the large numbers.
There is no doubt that parts will be ugly. A percentage of the people will not want to go where they are told. There is always a criminal element that takes advantage of chaos. A firm hand will have to be used to deal with everything.
But gathering them up is only part of the problem. The politicians of the various countries would have to decide how to ship them back. There has to be a political will to take action. The problem, as I see it, is no country wants to be seen as the bad guy. There will be riots. People will die. The alternative will be just as bad, or worse.
Mobs of people running uncontrolled around the country is worse than having the army take control. There is no touchy feely way to deal with the sheer numbers.