The U.S.'s debut game against Vietnam went down Friday in New Zealand, and a lot of people couldn't help but notice that most of the American players were choosing not to sing along to "The Star-Spangled Banner" while the famed music played before kickoff.

It was awkward -- the camera pans across the entire row ... and just about everyone, except for a few stray players, had their mouths shut. Some even had their arms behind their back.

The starters who opted to skip the Nat'l Anthem included ... Alex Morgan, Andi Sullivan, Sophia Smith, Naomi Girma, Emily Fox, Trinity Rodman and Crystal Dunn, among others. A couple were notably mouthing the words ... Alyssa Naeher and Lindsey Horan.

To make matters worse ... the Vietnamese players were pretty much all singing their national anthem, loudly/proudly it seems -- with hands over hearts. Ditto for just about every other international team we can see -- FOX is cutting all the national anthems on their social page.

On its face, it looks like every other country -- besides the U.S. -- is belting out their tunes.

It remains to be seen if the U.S. is going to do this every time they play another team -- the National Anthem will, in fact, be blared before each of their matches ... and since it seems like they're making a statement -- it has, no doubt, pissed off some along political lines.