It's like "300" but without the half-naked men...

[Polycarp]

3. Poison the town well and move around the army to try and deal with supply lines to force them to use the well water.

blinding sickness[/i] attack, here - disease is a far better way to take out a large force permanently.  Blind, strength-damaged hobgoblins won't pose much of a threat.  Disease is usually a pretty worthless attack because it takes so long to affect an enemy, but in this case it's just what the doctor ordered (so to speak).

Of course, that doesn't solve the problem of the ogres and trolls, but it seems unlikely that the heavies in the army would attack by themselves - and if they do, at least they won't have thousands of hobgoblins for support.

[Darkxarth]

Poison won't work.  It would be prohibitively expensive to buy enough doses of poison to successfully poison a 10,000 man army, and poison only strikes twice and then heals.  Unless it's Con damage poison that stands a reasonable chance of killing them, it would only be useful in delaying the advance or preceding an ambush - and it seems unlikely that you could ever afford enough poison to do any of these things.  I'm serious about the blinding sickness attack, here - disease is a far better way to take out a large force permanently.  Blind, strength-damaged hobgoblins won't pose much of a threat.  Disease is usually a pretty worthless attack because it takes so long to affect an enemy, but in this case it's just what the doctor ordered (so to speak).

Of course, that doesn't solve the problem of the ogres and trolls, but it seems unlikely that the heavies in the army would attack by themselves - and if they do, at least they won't have thousands of hobgoblins for support.

Sorry, yes.  Using Contagion to make the water diseased was what I meant.  I was just tired when I was writing that and thought saying poison was close enough.  x.

[Tybalt]

I'm running a war campaign myself. Here's a link to one of my threads on this site. There are some spoilers there, I'm not thinking you want to read the whole thing, but there are adventures in two of them: Siege of Fineberg Adventure Plots and City of Touchstone. (under adventures for the latter) There is also some background there.

http://www.thecbg.org/e107_plugins/forum/forum_viewtopic.php?24252

[Bill Volk]

How much about the opposing force do the PCs know? How much does the NPC general know? If nobody knows about those trolls, things will be a little trickier.

Conversely, what are the hobgoblins planning? What's their ultimate goal? How much do the hobgoblins know about the opposing human force? Do they have any plans to counter pesky adventuring parties who'd try to cut their supply lines and contaminate their water?

With no terrain to work with but open plains, this battle's going to be won or lost with intelligence.

[Darkxarth]

The PCs know:

1. There are 10,000 enemy troops.
2. Most are Hobgoblins, with Goblins and Bugbears thrown in.
3. There are a number of Ogres working with them as well.
4. There are two Trolls.
5. They will arrive at the border in 6-9 days.
6. The opposing leader is powerful, unusual, and mysterious.

The enemy knows:
1. The kingdom's army is significantly smaller (but not precise numbers).
2. The capital is heavily fortified, but the rest of the kingdom is easy pickings.

They know nothing about any particular adventurers, though the leader is sure to be wary of small parties sent by the kingdom, but I think the PCs are more than a match for any particular supply wagon or three.  They are pretty tough after all.

Also, thanks for the link Tybalt, those are good ideas.  I've looked at a number of them, but I'm going to go back and look at more later.

[AllWillFall2Me]

Here's what I would do. Attack them at their hearts.
Poor morale will devastate an army.

Have the rogue kill a few sentries every night, silently, and undetected. A thrown dagger, a fired arrow, any relatively minor attack should be enough to dispose of your average hobgoblin, when you add sneak attack. If they travel in groups, silence them, via the cleric. Or blind them. Have the cleric cast darkness and silence into various places in the hobgoblin camp, using the long range, and hidden vantage points.

If the bard speaks goblin, let him cast whispering wind to sow doubt among the troops. Imagine, a cluster of hobgoblins around a campfire, then everything falls quiet, then, once the sound returns, a whisper, almost inaudible over the fire, "The gods do not approve. <General's name> walks to his death. HE will die, as will all who follow him."

Then perform the town plan, with a magic mouth over the main gate: "Enter, doomed one. Judgement will find you. Its chosen warriors come."

Bit by bit corrode their faith. Have the PCs fight in a few battles, easily destroying hobgoblins, taking down a ogre, ad maybe a troll. Then, let the knight challenge the leader, one on one, assuming he can take him.
Assuming the knight wins, that army should crumble like feta cheese.

[Bromordra]

Hmm.. advancing army? Low powered standing army? Lots of standard issue commoners lazying about? two words: Mob Template.

If you have access to the Dungeon Masters Guide 2 then you know about the Mob template. If you dont, then look it up. It effectively makes 48 CR 1's that would be mere fodder alone into a CR 8 juggernaut that will plow through at least a few lines of Hobgoblins.

Now, you should only have to use this if the army actually reaches the city walls, otherwise, I suggest doing what the above posters said.

[Haphazzard]

Being big on using the terrain a flat landscape doesn't give me much.  However, I would suggest taking out supply lines.  I like the moral and disease ideas too.  Raiding parties.  I think that could be key.  I have two words for you: Molotov cocktails.  As much as I loathe disposing of liquor in ways other than being drank, a few molotov cocktails would do wonders to siege weapons, clumps of tents, and the like.  They're likely to have an armory with a large holding of bows and x-bows.  And what's an army without it's stomach?  Get a few raiders (less than 10) deep in camp at night and take out food stores and the slop tent.  Fire's cliche, I know.  But it sure does a good job of making things go away.  And it looks cool too!