This story was probably written in the brief period just after 1989 when Pons' and Fleischmann's cold fusion announcement had not yet been thoroughly debunked.
The story begins with a patrol by a mounted unit/company of the U.S. Border Patrol; they were using some kind of advanced (non-helicopter) VTOL flying personnel carriers. Specifically, they were powered by Pons and Fleischmann-type cold fusion powerplants. (Pons and Fleischmann were definitely name-checked in the story.) The viewpoint character notes that the U.S.B.P. has better and more advanced equipment (heads-up infrared overlays, etc.) and vehicles than the military.
The border (the U.S. - Mexico border in this case) is heavily militarized, with a wide free-fire zone which the unit is patrolling. Naturally the unit is attacked; I believe it was described as a gigawatt X-ray laser and the viewpoint character briefly tries to imagine how they must have hauled the massive generator for it up to the border by human power. The point of the laser is to disable the U.S.B.P. vehicles so the border can be breached. (I think the goal was to get a mass of illegal migrants across the border.)
I don't recall anything of the story after that. Just the advanced airborne (Pons and Fleischmann) cold-fusion-powered fighting vehicles and advanced combat tech, ambushed by a massive laser from across the border.