In some ways, modern battlefield technology (or rather, the advanced, cutting edge stuff wielded by the US, and to a lesser extent assorted others; after all, an AK47 and a dusty tarp are the most common battlefield technology in most of the world) is comparable to magic; wielded by specialist, it allows frightening mobility, situational awareness and firepower. An invisible, flying wizard with a wand of fireballs vs a stealth fighter; extended meteor swarm vs howitzers; spy satellites vs scrying or clairvoyance.
The point being, modern infantry are a lot like adventurers; tech savvy (magic savvy), well trained (lots of XP) fighters (in the genral sense, rather than the class) with top of the line gear, operating in small teams with complimentary skills. Of course, adventuring parties don't tend to team up as well as groups of soldiers do.
Real world armies haven't used lines of gun-wielding troops since WW I or before; air power and artillery make the tactic pretty much unviable. The same applies to armies of low-level commoners and warriors vs spellcasters; they only function they have is to distract the enemy while a competant assasin deals with them.
That's how it works in one of my homebrew settings, a magocrasy; rival wizards Evoke and Conjure the smeg out of each other, while their bodyguards and soldiers, small bands of Fighters (who all have spell resistance) defend their lieges, skirmish with each other and try to cut down the enemy mage.
A lord who insisted on sending levys against a high-magic enemy army would end up being very unpopular, and probably face revolt. As anyone who's GMed a party of 5th level or higher PCs, the damage they can do single handedly increases geometrically with level. After not too long, four high level adventurers cna do more than an army.
You may still need large numbers of troops for garrisoning and occupying, though; four heroes can conquer the city, but will have a slightly harder time controling them (unless they have a bard, I suppose).
Baah. I'm ranting and rambling, so I'll stop now.