Ok and where is the evidence the spread is significantly due to land border crossings? You really think the U.S. was being infected by Mexicans and Canadians? Refugees? That India is being infected by some or all the countries surrounding them? You must necessarily think these are illegal crossings because legal ones require COVID testing.

All it takes is a critical mass of your own population getting it, followed by wide scale gathering of millions of people with no meaningful mitigations. That's the obvious and simple cause.