The following year, the guerrilla army was opened to anyone willing to fight, a move that won the support of students, farmers and even some local bandits.

By NAJUA's 1937 heyday, troop numbers had swollen to more than 30,000, and its sphere of action encompassed more than 70 counties across Northeast China.

Earlier this year, Cao traveled to Baishan in Jilin to visit Huang Dianjun, a veteran who had been close to Yang Jingyu, a NAJUA commander.

When Cao arrived, Huang was lying under a quilt on a kang (brick-made bed) with only his long white beard showing. The 93-year-old's eyes were bright and gleaming, but he lost the ability to speak several years ago.

Huang's son said his father had often spoken about the difficult wartime conditions.

"In addition to fighting the Japanese army's encirclement and suppression, the NAJUA soldiers also had to fight nature," he said.

"He told me that in summer, the biggest threats were poisonous snakes and insects such as mosquitoes. The moment you realized a snake was hissing toward you was really terrible. In winter they had to walk in snow almost a meter deep, and many soldiers froze to death because they didn't have warm clothes or shoes," he said.

According to Wen, to cut the supply lines, the Japanese mopped up villages with connections with NAJUA, and forced the residents to leave their homes and live in camps. They also created a strip of "No Man's Land" and committed more than 500 massacres.

Baijia Puzi, a village in Tonghua city that was a NAJUA stronghold, was the site of one of the worst atrocities. The Japanese attacked in July 1936, killing 412 villagers. Only three people survived.