“What options do we have to stop this?”
The #1 priority of a standing army is to protect the nation from invasion. Any nation being invaded deploys its military on the border to repel the invasion. This is the solution. Otherwise, why have a military?
A nation that does not deploy its military to repel an invasion across its borders is committing suicide.
Sorry, but we are busy worrying over Ukraine’s border. No time for this stuff. Truth is, Putin could walk right across our southern border.