I'm not sure if it could actually be amended, per se, as this is the Bill of Rights, but it would definitely be circumvented once another country began saber-rattling. There are limitless possibilities for this: a permanently-federalized National Guard, an Army "Reserve" which is always on alert, "naval" infantry, the list goes on and on. No matter what happens, however, the Navy will come out on top, and Western expansion will be very limited: the US doesn't have an army to back up any claims. If the Texan Revolution does occur, there will be no American annexation and Texas will be completely independent.
Though that could be very interesting. The US provides arms and "advisors" for Texas, and in exchange Texas becomes our fighting arm. When the US demands a territory or a concession, it brings a Texan ambassador to the negotiating table. That would be a truly epic TL.