Zombie killer units sound ridiculous. The world hasn't gone full cartoon, just because the dead are rising. Most militaries are still employing traditional, albeit slightly modified, tactics against the situation. 

Alright, after doing some reading on the United State's military relationship with Japan, I've stumbled upon a lifeline for you. 

Seems the U.S has forces in Japan, owing to the whole World War Two thing, but they do not train the Japanese on U.S soil. However, I believe it could be argued that with the country falling apart, the Government would have pulled back everything from foreign bases to combat the rising threat of the dead. It could also be argued that they'd of probably been dealing with the dead in foreign lands, prior to the emergence of the shamblers on home soil. 

So with all that in mind, a U.S Naval Task Force sets sail from the Pacific, linking up with other U.S elements on the way, some NATO ones too. With them, they also bring a couple of JSDF destroyers - a symbol of international solidarity, given that the JSDF seldom deploys in foreign lands, as opposed to a full scale commitment.

The Task Force arrives in Africa during the early days, and begins operations in pacifying the mysterious disaster that is felling African states left and right. A few weeks into these, and it is quickly understood that the situation is beyond the Task Force's capabilities. Furthermore, the U.S starts reporting its own disturbances. So, it is ordered back the U.S. The NATO ships go their own way, to deal with problems in their own countries, but only one of the JSDF destroyers leaves - the other staying as a sign of solidarity with the U.S. At this point, no one probably thought the crisis would bring the West to its knees, so what's one destroyer?

The Task Force arrives on the East Coast, and is ordered to reinforce beleaguered bases and units. One such base is Fort Lesley J. McNair in Washington. The JSDF destroyer deploys its marines there, along with waves of U.S reinforcements, and places them under local U.S control. 

Fast forward to the present day, and Fort Lesley J. McNair has fallen to the shamblers. The UH-60's the survivors see were bugging out of there, and so it would assumed that the remaining JSDF troops tried to make it back to their destroyer - or another designated safe haven. However, their helicopter suffered an onboard outbreak, which resulted in panicked close-quarters firing. The American pilot is accidentally shot, and the helicopter crashes. 

The survivors can then come across the crash scene, and hey presto you're in.

Christ, that took some doing. Thoughts?