Just a thought -- if the Great Steamroller of the Wightpocalypse (or, for that matter, the Shadowpocalypse) is so easy to begin at 1st level, unless your campaign is set at the dawn of creation, someone will have already done it, which means either another someone has already discovered one or more points of failure in the plan or your campaign is a post-apocalyptic setting.
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I think that the "Tactical Spells" section having the picture it does is highly appropriate -- making the "fog of war" a literal battlefield presence is a useful tactic, if anything, even moreso at higher levels of magic. Everything from arrow fire to area-effect bursts is significantly degraded if you can't easily target your enemy. At low magic levels, equip the handful of low-level casters you can afford with wands of obscuring mist. At higher levels, control weather can arguably take care of a entire battlefield in one shot. Either way, the canonical War1-with-pointy-stick brigade becomes more relevant if they have some chance to actually close to melee before a rain of fire from heaven, much less a hail of arrows or old fashioned cavalry charge, obliterates them.
Also, in addition to Druid being crazy good at everything anyway, Druid spells are some of the best for the battlefield -- many of the biggest area-of-effect spells are there, and Natural Spell brokenation makes Druid nigh-undetectable scouts and weapons platforms (is your army seriously going to exterminate every squirrel and songbird they see?).
--On a more general note, unless armies have pretty high magic/power levels, PCs in most campaigns will pretty quickly outstrip all but the most elite commander/special monster units in raw personal power, and will generally be more effectively applied as a special-forces team than in a purely commanding role. (Not to mention most adventurer PCs are effectively Chaotic with respect to authority figures, much less military discipline.)
The additional advantage of a special-forces team (conscripted, mercenary, or otherwise) is that it is much easier to integrate into a more classic D&D style, in case your players don't want to play a purely military campaign the whole time. [Mixing your flavors can be really helpful -- I've been in, for instance, giant unending dungeon crawls with no RP/intrigue/etc breaks, and it gets really wearing after a while.]