CBI - China Burma India theater. As the
Japanese
spread out across China into Indochina (modern Vietnam and neighbors) they invaded Burma and threatened to invade India which was still an
British
Colony. The British had their hands full trying to stave off not only the Japanese, but deal with the internal threat of near revolution of the population of India who saw it as a chance for independence. America provided manpower, equipment and supplies to the Nationalist Chinese via the Burma Road and DC-3 flights over the mountains. The Office of Strategic Services (OSS, the forerunner of the modern CIA) mounted what was to be America's first attempt at Guerilla warfare in occupied Burma.
The long story short is that most people today do not realize that the groundwork of what happened in Vietnam was a direct result of the Japanese expansion into Indochina which was a french colony prior to WWII. To understand the CBI theater action you have to understand what was going on in that area of the world long before the Japanese invaded Manchuria.
As a side note, the American OSS was relatively successful in mounting that guerilla war against the Japanese using the many local tribes of the Burma countryside. Its too bad that they forgot the lessons and success of 1942-1945 just 20 years later when we blew it in Vietnam.