This would be because your team is already "at war" with the DL so that's why there is no 'attack' option.  If you are playing on lower difficulties, then your teammate would be drooling buffons and won't help much.  Even on higher diffculties the AI may be hesitant to send dhips to their death vs the DL lol.

Just a help to fight the DL, learn the missile line if you can (I forget what the tech tree islimited to in this mission) and send fleets of ships at the DLs.  They can fall to mass numbers.