Sixty years after the fighting ended in Korea, some soldiers who rallied to fend off the "American imperialists" are still seeking answers about China's involvement in the costly conflict. Beijing remains reluctant to declassify documents that might finally shed light on the decision to rush to North Korea's aid in 1950, resulting in the deaths of between 149,000 and 400,000 Chinese soldiers. Supported by decades of Communist Party propaganda, many veterans of the People's Volunteer Army (PVA) share the official view that intervention was necessary to protect China from US aggression. But others are sceptical. "All the cruel and bloody images I witnessed during the war are still vividly emblazoned in my mind," said Zhang Zeshi , who joined the fighting at 21 and was taken prisoner. "I regretted joining the war when I found out the US had no plan to invade China at that time." Zhang, who abandoned his physics studies at Tsinghua University to join the Communist Party in 1948, said China should at least tell the truth about "for whom it fought the war". "All humankind should learn lessons and work to prevent any wars from happening again," he said. The Korean war was the first major conflict of the cold war, pitting the communist North, supported by China and the Soviet Union, against the capitalist South, supported by a United States-led UN coalition. Although 18 countries participated, China has long known the conflict as the war "to resist United States' aggression and aid North Korea". The fighting started in June 1950, but China did not intervene until October, after the UN forces pushed the North Korean troops back to the Chinese border. The massive influx of Chinese troops - nearly three million in all - stunned the allies. The Chinese-backed North Koreans held Seoul for a time, but ultimately fought to a stalemate resulting in the 1953 armistice. Despite the lack of a clear victory, China's contribution to the war remains a point of pride for many veterans. They successfully challenged the world's most powerful military only a year after China had finished its own prolonged civil war. "We PVA are the world's first troops to defeat the US army," said Huang Zhao , 78, of Guangzhou, who joined the fighting when he was only 15. "We successfully punished the US, the world's superpower, and made the Americans respect us," he said. "The war was followed by decades of stability and peaceful development for our country." How many Chinese soldiers died in the war has long been a subject of debate. For years, China would only say that 360,000 soldiers were killed or injured, without providing a breakdown. Then, in 2010, Major General Xu Yan , a professor at the National Defence University, wrote and article for the People's Daily -affiliated Wenshi Cankao saying that 149,000 died and 26,000 were still unaccounted for. But Western sources estimate Chinese casualties at 400,000 dead and 486,000 wounded. "No matter how many people were killed, we shouldn't regret fighting the US because the war helped China win respect in the international world," said veteran Zhang Zhuan , 86. Similarly, questions remain about exactly why Mao Zedong decided to enter the war. East China Normal University history professor Shen Zhihua said Mao certainly wanted to help North Korean leader Kim Il-sung "liberate the South", but also wanted to win the trust of Soviet leader Joseph Stalin. Doing so helped him consolidate power in his fledgling republic. "Mao wanted to win Stalin's trust to show the two countries' treaty of friendship and alliance, which later helped him to win military, political, diplomatic and economic aides from Moscow," Shen said. In 1998, Shen published a book in Hong Kong relying on declassified Russian files to detail the ties between Mao and Stalin during in the war. It has been banned on the mainland. David Tsui, a North Korean affairs expert and son of a People's Liberation Army general, noted that Beijing received assistance on more than 300 technical projects after the war, helping the country to build up its industrial and defence infrastructure. Tsui, who is now teaching in Zhongshan University, also believes there was a secret deal between Stalin and Mao: if China helped push the US army beyond the 37th parallel and occupied Seoul, Stalin would help pressure the US to abandon Taiwan. How to parse different theories remains difficult as historical documents remain classified and those who were there are rapidly dying off. Beijing has little incentive to declassify documents that could muddy the legacy of Mao.