from CTH comment section:
All the President has to do is give the word….
Article 1 / Section 8
To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions;
https://constitutioncenter.org/interactive-constitution/articles/article-i
THE MIGRANT CARAVAN INVASION
What our president must do to seal the border.
October 22, 2018 by Joseph Klein
https://www.frontpagemag.com/fpm/271700/migrant-caravan-invasion-joseph-klein