The North Vietnamese military (also known as the Vietcong) tactics made it difficult for the United States to defeat them because:
A) They did not wear uniforms- This made it challenging for the United States military to differentiate between who was a citizen and who was a member of the Vietcong.
B) Ambush/Hit and run style of fighting- This military style of fighting can be seen as unpredictable and random. This makes it so that the US military has to be on high alert at all times.