@Switch BlaydeI just googled the question asking what Chicago Manual of Style would do (the article probably followed the AP Style guide, but I follow CMoS). Google once again invoked its AI and said:
Yes, the Chicago Manual of Style says to capitalize the word "army" when it's used in works about the US military, even when it's standing alone. For example, "the Army".
However, the Chicago Manual of Style says that "army" and "navy" are considered generic when used alone. For example, "the two armies were in position". In contrast, "Army Corps of Engineers" and "Marine Corps" are capitalized because they are more specialized subbranches of the US military.
The military's own documentation routinely capitalizes all these terms.
I asked the same question replacing Chicago Manual of Style with AP Style Guide and got:
Yes, "Army" is capitalized in AP style when referring to the U.S. military branch
So "army" should have been capitalized in the article (not the US Army, but the Mexican Army).