The world of The Walking Dead was doomed well before the iconic lead Rick Grimes woke up from his coma. The Walking Dead's zombie apocalypse - inspired by the early work of George A. Romero - treats Walkers as more of a natural disaster to navigate than enemies to kill. Walkers are mostly dangerous when they surprise humans, or when enough of them congregate together to create a 'herd,' making life post-outbreak about learning to manage the undead. However, while later survivors would figure out smart ways to cope with Walkers, they were badly mishandled in the very early days of the outbreak.
Way back in The Walking Dead #2 by creative team Robert Kirkman and Tony Moore, Rick found a band of survivors led by Glenn, after nearly being killed by a herd in Atlanta. When Rick mentions that he's never seen so many Walkers together, Glenn explains that cities quickly became death traps because of a fatal decision made by the government. They herded citizens to big cities in an attempt to set up human bases that would be easy to protect. However, due to the nature of the outbreak, concentrated areas made it easier for Walkers to spread. This was a major error on the government's part to protect people, but not just because it sped up the rate of infection.
Walking Dead's Harshest Zombie Rule Doomed the Government
The government clearly thought it could prevent any incoming Walkers from attacking anyone inside the cities by making barricades and perimeters to keep citizens inside safe. However, the government missed something crucial. It's later discovered that Walkers aren't actually created by being bitten, as with zombies in other media. While it may speed the process up, everyone is already infected and will turn into a zombie if they die (short of their brain being destroyed in the process.) Even death from illness or person-to-person violence causes the victim to rise again. Because of this, the threat was always going to come from the inside, and gathering people together only made this harder to manage. Had people been more spread apart, random deaths could be managed individually (as in Walking Dead's eventual post-outbreak future), but among millions of people packed together, one Walker quickly creates more. However, there's yet another reason this was a terrible idea.
The Government Accidentally Created Zombie Herds
In Walking Dead lore, herds are the ultimate form of zombie threat. Once zombies end up in the same place, they tend to stick together, and over time this can create a vast sea of the undead. While individual Walkers are easy to kill, thousands of them at once can overrun human cities simply through numbers - pushing down walls and cutting off all escape routes. As the series progresses, Rick runs into some truly vast herds, and has to work to divert their path since there are simply too many undead to even consider killing.
By gathering people together, the government guaranteed that these huge herds were formed immediately, creating unbeatable zombie herds which then wandered free across the unprepared country. Had this process been delayed by spreading people out, America would have found it way easier to defend against or manage the same number of zombies, having the chance to whittle away at them in smaller groups.
America Fell Quickly Because It Misunderstood the Infection
Of course, this isn't to say that people like the president had any intention of letting things go badly. Since the cause of zombification was still unknown, it was an idea that seemed good at the time since they had the firepower to prevent any Walkers from getting in. However, that's simply not how The Walking Dead's infection works. Theories have estimated that Rick Grimes was in a coma for approximately a month, and the country is all but destroyed when he awakes, showing how quickly the government's plan failed. The world of The Walking Dead was doomed from the very start, and all it took was one fatal, well-intentioned choice that misunderstood the nature of the undead threat.