They can prevent them from crossing. How?
Will they shoot them?
Will they push little kids with their mommies down the hill back into the river?
Imagine how you think they are going to keep these people out of the country. And then envision it on CNN. Fox even.
The knee jerk, lets use force answers are not going to work.
One thing is certain, this scheme of throwing together a mob of immigrants while planting isis members within them is going to backfire on the anti-American, anti-Trump groups.