Country A under GDI
Country B under GDI

Country B never hit country A
but
Country A double tap Country B
If Country B send a missile to Country A ( first and only hit )

Would Country A allow to GS / BR / AB Country B??

at express/primary/tournament you can attack anyway you want.

Marshal obviously doesnt play Express as he dont know about the Express GDI.

and the answer is yes, all attacks would be allowed on both sides once the missile is sent