The issue needs to be de-militarized.
One pathetic refugee does not make for an 'invasion'. This is not a principle but a practical matter. We can afford to accept refugee claimants. Whatever these border crossings are, the legal status makes them difficult to deal with. We could patch that hole by humane means though.
It's not that simple. The legal system makes everything complex. Yes, one refugee does not make an invasion. Thousands and thousands of them can. Trump can bluster all he wants, but just like Canada, if they walk up to a border crossing and claim political asylum they can't be turned away. Then the lawyers get at it. Right now in Canada, the first hearing an asylum claimant is being offered is 2 years away. Then you have the immigration board appeal, then it goes to the federal courts, and five or six years later they're still here on social assistance and have had a couple of kids, and now what do you do?
As I said in another post, this is only going to get worse. If, as the climate types suggest, the earth gets hotter, we're going to see caravan after caravan. Potentially tens of millions of people migrating to the north. Do we let them all in even if it destabilizes us and destroys the welfare state?