So, the players aren't going to be in the army sweeping the world, eh? Well, a few ideas for players fighting against the tide (all these are going to assume a monstrous army, change it up as you need):
Start small. Just give out rumors during the first adventure. Maybe hearing reports of heavy "bandit" activity near the border, or trouble with war in a far country. Read the beginning of Dragons of Autumn Twilight for an example. Lvls 1-2.
Slowly increase the level of activity. Send the heroes to check out what happened at a border fort that hasn't reported in a while, or to check out rumors of "Bandit King" in the far reaches of the kingdom. Leave hints of a more organized force, but nothing that could act as solid proof.
The players find a small force of hobgoblins occupying the border fort, with a dark cleric raising the bodies of the dead guardsmen. Helping the cleric is a fighter in the uniform of a neighboring country , or missives in the cleric's effects from a king of a hobgoblin tribe, stressing the importance of this first strike against the player's home. Either way, make sure to drop the name of the BBEG the players will be meeting at higher levels, but only use it once. Maybe the fighter calls out the name as he joins the fight, or the message ends with "In the name of "BBEG". Lvls 3-4
Next, give the players a taste of command. Maybe they find the remnants of a legion that was out on patrols, and they have to help them to hold back a force of goblinoids attacking the border (way too many for the players to kill off, themselves... at least 1,000). The commander was killed in the initial combat, and now the officers need someone to look up to.
Whatever route the players take to help out the legion (sneak into the enemy camp to kill the leaders, use guerrilla tactics to winnow down the goblinoids), at the moment of definative victory, a scout rides up to announce the arrival of the main army (the goblinoids were just the vanguard), several tens of thousands strong. At this point, you should reveal who the person responsible for sending in the army is, as divinations are going to reveal it anyway, once news of the army reaches the capital. This will probably be a very charismatic Hobgoblin/Orc/Giant General. Either way, this general is just a pawn, but don't reveal that to your players unless they work very hard at figuring it out. Lvls 5-6
Now the future starts looking dark. The peasants are flooding into the cities to avoid the war, which is causing starving, large ghettos of refugees devoloping in the cities, etc. Give the heroes missions from the king/regent/high priest, such as stopping a half-fiend wizard who is summoning demons in the capital, or requesting help from the elves in the dark forest. This is the point where the pc's homeland starts fielding their army, slowing the advance of the Enemy. At this end of this phase, the players should have quite a bit of autonomoty (sp?), and should be able easily request audiences with the king. Include references to the BBEG alot here, just not enough to directly link him to the army.Lvls 7-10
The future should look bleak for the heroes' homeland now, as the Enemy closes around them, seemingly unstoppable. The pc's should have an impact on the Enemy, but there's just too many of them...
Each adventure should be focusing on negating an advantage of the Enemy, or countering their own disadvantages: Leading a large force in a counter strike against the floating citadel providing the necromantic energies to an entire army of undead; or helping the dwarves to activate hundreds of golems, but to get the key, they have to travel through the dwarven land of the dead. The pc's show up on Legend Lore now, so make their actions should be epics in the making. Let them shine once in a while. Lvls 11-13
Finally! The players are in a position to deal a critical blow to the Enemy. Now's the time for them to strike back against all the hopeless and oppression that's been hounding them for the last few phases of the campaign. This is the point where they should be storming the headquarters of the general/high priest/archwizard leading the army. Let them kick booty and take names, make the final fight long and hard, and give them time to think they've earned their retirement... then hit them with the knowledge that the BBEG was just using the general for a pawn... and now he's gunning for them! Lvls 14-16
And at long last, unleash that demon prince/arch lich/whatever you've always wanted to use. The pc's are now commanding the highest levels of magic and might the world has seen, so throw it back at them. Make these powerful adventurers stop the ritual Orcus is preparing (which would turn the world into a hollow husk filled with undead), fight off the Tarrasque, and otherwise save the world. Make the end hard, and your players will remember the campaign forever! Lvls 17+
Wow, that went on alot longer than I intended. Anyway, hope it helps!