As the end of World War II neared, several Japanese Imperial Army units were cut off from communication with their home bases. Some did not receive the surrender orders in 1945, and indeed, fought on for several weeks after the official surrender.
A few hardy individuals, though, held out for several decades, shunning contact with the "enemy", ekeing out an existence in the jungles of the South Pacific.
The first forgotten straggler found, Sgt. Shoichi Yokoi, was discovered by a hunting party in the Fena mountain range on Guam in 1972. He had been living there for over 28 years, after he was separated from his unit during the American attack. His rifle had rusted into uselessness, but he used his skills as a tailor to fashion clothing from native plants and fibers.
Sgt. Yokoi's discovery sparked searches for more Japanese stragglers; in 1974, after being pursued for several months by local police, Lt. Hiroo Onoda surrendered to authorities on Lubang island in the Philippines. He had originally been sent to spy on American airfields, and kept himself hidden for over 29 years, after his unit committed suicide when the US Marines landed. Unlike Yokoi, he and another soldier (shot by police in 1973) kept their rifles in working condition, and allegedly killed 30 people, wounding another 25, during their 29-year stay on the island.
Facts gathered via google.com, Asiaweek, and Sgt. Yokoi's obituary at http://vikingphoenix.com/public/rongstad/bio-obit/yokoi.htm.