Just the guerrilla warfare. The NVA were conventional troops, as were the US troops, and therefore weren't quite as bothersome. The NVA & US Army fought under similar rules. The VC did not.
Had the US continued to utilize Green Berets/SEALs etc. in lieu of conventional US troops, the Special Ops men would've been able to continue to deal with the VC guerrillas. As that was their specialty.
But conventional US troops (regular troops) found the VC to be annoying.
When the VC surfaced during Tet, that settled that; they were nearly wiped out.
the weather
The existing central American population
General George S. Patton's comments on the "Soviet bunion" reflected his strong anti-communist sentiments and desire to confront the Soviet Union after World War II. His remarks, which suggested a more aggressive stance against the Soviets, caused difficulty within the U.S. military and government, as they contradicted the official policy of cooperation with the USSR during the post-war period. Patton's outspoken views created tension among military leaders and policymakers, complicating diplomatic relations and undermining the collaborative efforts needed to stabilize Europe after the war.
Military conscription, which caused protests and riots.
It was a period of financial difficulty caused by the 1929 Wall Street Crash that led to a global depression. Britain had an economic decline
This has caused me great difficluty.
the weather
Cheap energy, military spending, and rising productivity.
volcanic activity caused the most destruction in pompeii
showing audience charts and maps of military strengths
In the 17th Century, the Spanish Crown had difficulty suppressing a revolt in the Netherlands because of the difficulty in raising an army.
Human activity
acxs
Volcanic activity.
http://www.iraqbodycount.org/ says about 100,000 which includes suicide bombings, assassinations, car bombings and from American military action.
because it is caused by too much physical activity
Fatique