into the countries of Laos and Cambodia, because the Viet Cong were crossing the borders of these countries (Vietnam had borders with China, Laos and Cambodia.) The Gulf of Tonkin Resolution passed by the U.S. Congress officially began the war, and after Nixon launched a ground invasion of Cambodia in 1970, the Congress repealed the Tonkin Resolution and passed the Cooper-Church Amendment which prohibited U.S. troops from operating outside of South Vietnam. However, from the beginning of the ground war in Vietnam, the military command surreptitiously ran Special Forces operations inside the Cambodian and Laotian borders to interfere with the Ho Chi Minh Trail supply lines that traveled from North Vietnam through both countries. A look at a map of Vietnam will easily explain this.
The bottom line is that it wasn't illegal for Kerry or anyone else to be in Cambodia or Laos, but our government was lying about our policy to the American people and the outside world. It didn't become illegal until the Cooper-Church Amendment passed in 1970, following the Kent State massacre and the widespread campus riots.
There's a general overview of the entire Vietnam War on Encarta and the following refers specifically to Cambodia:
http://encarta.msn.com/encnet/refpages/RefArticle.aspx?refid=761552642#endadsIn March 1969 Nixon ordered the secret bombing of Cambodia. Intended to wipe out North Vietnamese and NLF base camps along the border with South Vietnam in order to provide time for the buildup of the ARVN, the campaign failed utterly. The secret bombing lasted four years and caused great destruction and upheaval in Cambodia, a land of farmers that had not known war in centuries. Code-named Operation Menu, the bombing was more intense than that carried out over Vietnam. An estimated 100,000 peasants died in the bombing, while 2 million people were left homeless.
In April 1970 Nixon ordered U.S. troops into Cambodia. He argued that this was necessary to protect the security of American units then in the process of withdrawing from Vietnam, but he also wanted to buy security for the Saigon regime. When Nixon announced the invasion, U.S. college campuses erupted in protest, and one-third of them shut down due to student walkouts. At Kent State University in Ohio four students were killed by panicky national guardsmen who had been called up to prevent rioting. Two days later, two students were killed at Jackson State College in Mississippi. Congress proceeded to repeal the Gulf of Tonkin Resolution. Congress also passed the Cooper-Church Amendment, which specifically forbade the use of U.S. troops outside South Vietnam. The measure did not expressly forbid bombing, however, so Nixon continued the air strikes on Cambodia until August 1973.
Three months after committing U.S. forces, Nixon ordered them to withdraw from Cambodia. The combined effects of the bombing and the invasion, however, had completely disrupted Cambodian life, driving millions of peasants from their ancestral lands. The right-wing government then in power in Cambodia was supported by the United States, and the government was blamed for allowing the bombing to occur. Farmers who had never concerned themselves with politics now flooded to the Communist opposition group, the Khmer Rouge. After a gruesome civil war, the Khmer Rouge took power in 1975 and became one of the bloodiest regimes of the 20th century.