I don't think anyone really "failed".

If it was a planned international attack, it could have been something as simple as a suitcase bomb, or even a single vial of liquid(e.g. T-Virus). Once the "virus"(which would have had to been airborn) is released, i don't think anyone could have prepared for it. Plus i honestly don't think it would be impossible for a group to plan something as simple as a small simultaneous attack with a hand held bomb/vial. By the time the intelligence agencies noticed something, started an investigation, and tried to stop it, it could already be too late.

Maybe it was something natural....like an earthquake(cracks in the ground) releasing some type of gas that's causing it.

Either way, both things would have been very difficult, if not impossible, to prepare for or make a stand against after the fact.. As fast as the infected turn, it would take no time to spread because of a few main factors. First, the zombie apocalypse would be so unbelievable that many would simply fall victim to hesitation. Second, the population in big cities is so high that it could infect thousands in a matter of minutes like a big domino effect. Third, Think of the lack of personnel due to awol to be with/find family during the apocalypse. Fourth, think of how many ground troops it would take to halt the spread of something like this. It would be impossible.