lord of the rings - Did the Army of the West know why they were marching on the Black Gate?

From the point of view of common soldiers, it might not have been a "suicide mission". Aragorn had just led the Dead Men of Dunharrow to destroy the army attacking Minas Tirith when all seemed hopeless. When they asked for volunteers to march on the Black Gate (as per the quote in Wrathchild's answer), the soldiers might have believed Aragorn knew what he was doing, and expected some sort of magic to save them.

In fact this was basically correct, although the soldiers didn't know about the Ring and were probably expecting something more like the Dead Men. Probably Aragorn just chose not to discourage any hopeful rumours which were circulating in his army.

After all, which bit of dialogue is more likely? This:

SOLDIER: My lord Aragorn, will the Dead Men come to save us again?

ARAGORN: No. And we will be hopelessly outnumbered by the armies of
Mordor. But if we're really lucky, a halfling will drop the Enemy's
Ring into the fires of Mount Doom before they finish hacking us limb
from limb.

Or this:

SOLDIER: My lord Aragorn, will the Dead Men come to save us again?

ARAGORN: [Smiles slightly] I cannot say, but there is much
more to this strategy than our army marching on the Black Gate. All
will be revealed when the time is right.

Also, the soldiers might have thought the appearance of the Eagles was Aragorn's secret weapon, which would have helped their morale at a crucial moment.