"We wait for their attack."
Translation:Kami menunggu serangan mereka.
"Kami tunggu serangan mereka" should also be accepted as it doesn't change the meaning and is also grammatically correct.
Actually, "kami MEnunggu serangan mereka", just slightly implies that "we ARE WAITING for their attack", as it is more of a (i guess) "perfect" tense congruent with the Engish perfect tense.
Although, in Indonesian there are no "tenses", and therefore both versions should be correct and accepted.