Why is China Still One Country?

You slavering lefty dogs have gotten me wildly interested in the history of China and it's progression into Socialism (Communism? Chineseyism?)

What I see over and over is regional warlords resisting eachother, regional powers fighting for dominance, etc. I'm incredibly fascinated. But what has really struck me is that through all of that, China has remained one nation, with essentially unchanged borders. Reading at a surface level, it kinda feels like China should have splintered into several countries somewhere along the way, a la Europe after the fall of the Roman Empire.

What, in your opinion, prevented China from balkanizing over the centuries - and particularly from the mid 1800s through Mao's revolution?

Was it the continued presence of imperial powers? Did England, Japan, France, America, etc. provide a continual backstop, an opposing team, that kept the competing factions from totally splintering?