Stepping a bit back but... no-way, no-how.
I think you vastly underestimate the logistics behind extending force. Those people have to be moved, supplied with gear, and, most importantly, fed.
This Thanksgiving, it is estimated that 42 million Americans will travel 50 miles or more from home. And that travel is distributed over the entire nation, not all at one spot - it is spread through all the planes, trains, and highways, and it'll still be massively crowded.
Now, try to move more than double the number of people, and put them all in one spot, so most of them are traveling a lot more than 50 miles. There is no place on the planet set up to handle that kind of influx in anything like short order. In addition, continuously move a hundred million gallons of water and a three hundred million pounds of food *EACH DAY* to those people. And, get all these people their guns, and medical care for when they've been rent asunder by an ogre...
Oh, and move and feed all the people to support the fighters, as well.
No. That's just not happening.
You are vastly underestimating the world's capability for organization, movement, and discipline. You're probably imagining a massive crush of people, milling about and throwing ammunition at each other, but it's really more of clockwork.
You'd have people organize into their units, standing in lines to receive their gear, their food, their weapons, their water, sitting through breifings, getting into airplanes, and going where they need to go. Very orderly, very boring, very organized.
"But it's a lot of units all at once!" No, it sure isn't. It's one unit at a time, through redundant military airports, processing stations, armories, and centers. First the recon units, then the base constructing guys, then the security elements, then the main army force, then you rotate each of those units out.
Now, I don't know how difficult it would be to move 100 million people, per se, especially considering that the US' military strength is 1 or 2 million people. The real point is that it's entirely possible for everybody in the world, from every country, to field 100 million people. I can't think of a single reason we'd need 100 million people around, especially when we're dealing with high-explosive rounds, ICBMs, tanks, artillery, ship-board bombardments, aircraft and aircraft carriers, and the like; or when you consider the fact that the US alone probably has enough conventional explosives stockpiled to turn a medium-sized country into glass.
But I'm pretty sure it could be done.
The reason that Thanksgiving is such a mess is that it's totally disorganized, with people taking less optimal routes, not considering the travel plans of others, and travelling down a totally decentralized system and going who the hell knows where. When you organize properly, you get a lot more done, and that's kind of the one thing the military is good at.
I could go into it more, but I'm already afraid I've gone rather into more depth than I'd intended.