Or­ange no-en­try signs post­ed by the U.S. mil­i­tary in Eng­lish and Span­ish dot the New Mex­i­co desert, where a bor­der wall cuts past onion fields and parched ranch­es with tufts of tall grass grow­ing amidst wiry brush and yuc­ca trees.

The Army has post­ed thou­sands of the warn­ings in New Mex­i­co and west­ern Texas, de­clar­ing a “re­strict­ed area by au­thor­i­ty of the com­man­der.” It’s part of a ma­jor shift that has thrust the mil­i­tary in­to bor­der en­force­ment with Mex­i­co like nev­er be­fore.

The move places long stretch­es of the bor­der un­der the su­per­vi­sion of near­by mil­i­tary bases, em­pow­er­ing U.S. troops to de­tain peo­ple who en­ter the coun­try il­le­gal­ly and side­step a law pro­hibit­ing mil­i­tary in­volve­ment in civil­ian law en­force­ment. It is done un­der the au­thor­i­ty of the na­tion­al emer­gency on the bor­der de­clared by Pres­i­dent Don­ald Trump on his first day in of­fice.