That was one labored sentence! I cannot be the only one that parsed it as: “Trump says he’s been rejected by NATO (while) most allies (are) to join mission …”

Instead, it wants to say: “Trump says he’s been rejected by NATO and most allies (in his desire for them) to join mission …”

The comma is for enumeration and not for segregation of two sentences. So it makes sense. They just used a comma instead of an “and”.