Can't launch attack

[Suicide Junkie]

Also: even if you have the option set, you won't see the blue path line when giving an order to attack. (unlike with a move order) This would be because the path to take is unknown, since the enemy may move as your ships chase it down.

[Gil Hamilton]

I had a fleet not attack once, as well. Still had movement left. As it was to be the opening salvo in a campaign to finally get rid of those pesky guys, I was a little curious.
I immediately sent a declare war message on the neutrals (we had a trade agreement at the time). Next turn, my fleet moved in a mopped up on 'em! Maybe you can't just start shooting when you have a treaty with them...

[dumbluck]

Correct. Any treaty above non-intercoarse means that you can't fire on them, they can't fire on you. This applies to units, as well (ie, mines). No more treaty, no more limitations on targets.