DM needs advice before session at 10am (currentely 1:22am)

[Orsen]

A party of five tenth level. Wildshape Ranger, Swordsage, TWF Barbarian, Psychich Warrior, Cleric Archer.
Long story short:
I just fiinshed playing with a group and we are continuing in the morning. I'm DM. My party was sent to investigate a cavern where a Drow army is building up but they don't know it. They were supposed to get in, fight through the advance guard see the army then report back. Instead they triggered a trap a bunch of times and the Swordsage went to a strength of 0. This caused them to run from a Beholder and hide in town for 11 days for the Swordsage to get his full strength back.
I had already planned in my notes for the Drow army to start leaving in 7 days and be completly out in 9. Now the party are traveling further into the cave chasing after an escaped Drow who was a guard for the underdark Drow city the army massed in. Basically the party is walking into a Drow city also full of Mind flayers (both are attacking the surface) and since they are underground, the players have no idea that the countryside is being attacked by a Drow army.
How do I get them to not walk into the city and be utterly overwhelmed by numbers or how do I make them not all die and remain true to what's going on outside the players knowledge?
Thanks in advance to anyone who has a suggestion!

[Embermage]

They are heading into the caves through which an army of drow just came out, right? That army should leave a hell of a mess behind. Have them pass clusters of cooking fires, trench latrines, and other obvious signs that an army passed this way. Even elves are bound to leave some tracks in a group that large.

Also, you could have a handful of smaller drow camps and settlements around the outskirts of the city. With the army on the march, those settlements are left undefended, and the inhabitants have retreated to the safety of the city. Leave signs that they didn't just vanish; they had plenty of notice that they would be leaving, so their belongings would be packed up and taken with them.

If all else fails, you can have the PCs run into a neutral/friendly NPC on their way in, someone who did manage to see the army of drow assembling. A drow army on the move is sure to be the hot topic of underdark conversation.

[Kaun]

If there going in throught the same cave that the army exited throguh have them find the tracks or the remains of a camp that the army used to try and convey the forced that moved through.

If they head onto the city let them have a look at the outer walls of the city befor they get to close and try and convey how deadly it would be to go in.

If they do it anyway then so be it.

You could all so have them run into an drow scouting party on the way back to the cave and have one of the drow let info slip about the army.

[Argok]

You could have them find a spec ops team(a better name is needed) that is going to attack the city and make it look like a advance party so the city tries and sends out news to the Army to come back. Have the spec ops team ask the party for some help.

Or have the city be sealed. Just have large amounts of tunnels to wander through. Maybe some other monsters to fight. Bats are normally in caves.

3rd option. Have a drow attack them from behind. Upon beating him. Have a note that says something about a coup in the army. That leads to it's own adventure. Since drow all want to be better then those around them. So you get temporary allies that change quickly when they think the party isn't useful.

4th. Giant train stops in front of them. They get on(or forced on). It drives off into the sky. where rocks fall and everyone dies. Oh and the train says "I think I can". Over and over again. Yelling at anyone who would interrupt it.

[Orsen]

Thanks so much everyone! I'd already done a bit to try and make it show that an army had gone through but just not nearly enough. I'll definitely use some of these ideas and hopefully the party gets the message.

[TitusAndronicus]

I realise it's a bit late to be helpful in this situation, but the Three Clue Rule seems sensible to me. It definitely makes sense to me that players will probably overlook or misunderstand one clue, no matter how obvious it is.

[Madwand99]

11 days? Did the cleric not realize he could cast Lesser Restoration or Restoration? At 10th level, a party should never be out of things for more than a day unless they are crafting or something.