Sci fi: 1970s/1980s all I can remember is bar scene involving protagonist last chance to make it

[Carlo]

At the start of the book the protagonist is failing his exams to make it into the military, and he doesn’t know his last chance is at a bar in Switzerland where someone keeps pestering him and he manages to pass his psych by grabbing the assailant by the wrist and asking him something like “Can I help you” in his bad German. IIRC correctly if he had failed this last test it might have had dire consequences

[FluffyMonkey]

Do you remember what made it sci fi?

[Carlo]

It feels kinda like the forever war by Haldeman but I remember something about aliens coming through portals and time travel

[Daisy]

This is an extremely long shot, but your description reminded me of the end of Mostly Harmless.