As I understand the dynamics of the Walking Dead universe (at least as illustrated in Fear the Walking Dead), the apocalypse starts when a wave of “the flu” sweeps through the country — except this flu is rapidly fatal if you’re symptomatic. Note that several people “turn” without being bitten. The sickest members of society die first (junkies and the homeless), but then people start falling like flies within 24 hours or so of the first victims. They immediately reanimate and start munching everyone around them.
The immediate chaos would impact our military almost as badly as the civilian population. All of a sudden guys are fighting off zombies in the barracks, officers are home trying to keep the wife from eating the children – all while they’re being frantically called in to collect their weapons and respond to the nightmare that’s erupting everywhere. In a nation of 320 million, if even 5 percent are fatally infected with the initial virus, that’s 16 million zombies munching away, plus a significant and immediate loss of military combat power as troops fight off their undead comrades (not knowing that every bite is fatal). Assuming the military could stabilize its own bases within, say, 24 to 48 hours (though the barracks, dining facilities, and hospitals would be scenes of utter carnage), then the few hundred thousand remaining troops (most of them not combat soldiers) would face the task of dealing with a zombie population that would be crossing the 100 million mark, and rising.