There were a number of elements that made fighting in Vietnam difficult. Among these were thick jungles, lack of acclimatization of occupying forces (French then US troops), difficulty identifying the enemy (a guerrilla force that blended with the homogenous population), lack of home front support (growing social unrest and antiwar sentiment), unclear objectives, ineffective (puppet) government in South Vietnam, unsteady ally government, long supply lines (approximately 8,000 miles of ocean between the US and Vietnam, and a host of other reasons.
british
Some of difficulties the continental army faced was a shortage of food, clothing, and sickness after the army camped in Pennsylvania in December 1776.
The North Vietnamese governments guerrilla fighters and regular army.
North Viet Army (NVA)/Viet Cong (VC) vs Allies.
They didn't, the British were in Burma & India fighting against the Japanese Army during WWII.
After fighting the most powerful nation on earth for over a decade, the NVA was by far the most hardened Army in SE Asia.
The us slowly pulled the troops out and handed the fighting over to the south vietnamese army
Yes, Republic of South Korean Servicemen fighting in Vietnam were referred to as ROK's...the ROK Army (Republic of South Korea).
The NVA and the VC (North Vietnamese Army and the Viet Cong).
The United States where fighting the North Vietnamese Army to deter communism in South Vietnam. The Chinese also were a part of the conflict however not so much as a direct entity as a funding source for the NVA.
US Servicemen were fighting regular (NVA) soldiers and airmen from the North Vietnamese Army, Air Force, and Navy (Tonkin Gulf Incident). US Servicemembers are NOT fighting the Iraqi Air Force, Army, or Navy.
The same enemy everybody else was fighting: the North Vietnamese Army (NVA) and the Viet Cong (VC).