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North Korea
The US is trying to establish law, order and stability to Iraq. WW1, WW2, Korea, and Vietnam were wars against a foreign nation. The US is NOT AT WAR with the nation of Iraq; we are policing it; arresting and terminating terrorists, etc.
Vietnam and Laos. North Korea and China are also communist but are not in South-East Asia.
First of all the classification of the word "war" will have to be agreed upon. WWI, WWII, US Civil War, Spanish-American War of 1898, Korea, Vietnam, etc. were all Fought against a nation. Those nations being: Germany, Japan, Spain, North Korea, North Vietnam, etc. In "Operation Enduring Freedom" in Afghanistan...the United States of America is NOT at war with Afghanistan! Therefore, the "longest war" against what nation???
Yes, South Korea is an ally of the Republic of Vietnam. During the Vietnam War, South Korea helped the United States and the Republic of Vietnam to fight against the communists. The Republic of Vietnam was helped by South Korea, by the United States, by Thailand, by Taiwan, by New Zealand, by Australia, by Singapore and by Malaysia!
Mesopotamia, Egypt, Vietnam, North Korea, South Korea, China, India, Thailand, Cambodia, Greece and Italy.
In the Vietnam War the US opposed the invasion of South Korea by North Korea. The fighters from North Korea, who used guerilla tactics, were known as the Viet Cong.
Australia, New Zealand, South Korea, Philippines, Thailand, and South Vietnam.
US, Australia, New Zealand, Philippines, Thailand, S. Korea, S. Vietnam against North Vietnam and VC.
"Communism" China, Cuba, Loa People's Democratic Republic, Vietnam, and North Korea
The 2nd war against communism (Korea was the first).
US, Australia, New Zealand, S. Korea, S. Vietnam, Thailand, Philippines against North Vietnam.