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.
guerrilla tactics, striking unexpectedly at American troops
the weather
The existing central American population
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
Tecumseh's death caused the Native American Confeseration to fall apart.
This has caused me great difficluty.
the weather
volcanic activity caused the most destruction in pompeii
Cheap energy, military spending, and rising productivity.
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.
because it is caused by too much physical activity
They were created by volcanic activity.
Fatique