It's all about travel speed.
In the past, troops and information were pokey. In the distant past, troops moved by foot or by horse; for most of the last century, it took trucks and ships to move a large amount of firepower. And until recently we didn't have the Internet or satellites, either.
When fisticuffs between two nations did break out: first off, other nations didn't even find out about it for a long time, because the Internet and satellites didn't exist. We couldn't just photograph Europe to find out Germany was at war against Poland. We had to wait for somebody to get out of Poland (or send a cable) and alert the rest of the world that there were shenanigans going on. And then we didn't have a complete picture of what was happening on a national scale--the guy who got out of Poland could only say "I saw shooting and bombs being dropped here." Without satellite photography it took a while for people to assemble a nationwide picture of what was happening. And once everybody else found out that Germany was in fact invading all of Europe, it took a long time for everybody else to get significant numbers of troops in there to fight back.
So in the past, when a war did happen somewhere, it was usually months or even years before anybody else could do anything about it. That's all changed. Today, five minutes after something happens, we see it happening. Courtesy of Google Earth. And troops can be moved much faster as well. Fifty years ago it took a couple of months to conquer a nation. Today, air power can cause severe infrastructure damage within a matter of hours (whereas half a century ago an entire nation's air force needed weeks or months to cripple another nation), and complete ground superiority can be established nationwide in a matter of days.
Today, the strongest nations of the Earth can blow up pretty much whatever they want, whenever they want. It's pretty difficult to start a war against somebody when all their allies can jump right in and shoot back five minutes after you pull the trigger. That's why all of today's wars are little ones: the only way to get a war started is to be small enough that the developed world doesn't give a crap. Which currently seems to be the case in Syria. (cue generic accusation of hypocrisy against the world community for going to war against Libya but not against Syria)