In the old game wasn't it that an ally would send 25% of available units, but no more than 25% of the actual defending country itself? Maybe I've been remembering incorrectly this whole time.

Then a country with only 1 turret cannot use 500k of his allies 2m turrets for example.

Yeah, was probably me. I was stunned by one failure the other day I simply had to test again with a higher ps.