All of this stuff must surely be in your textbook -- there must be something you are expected to say. That's because it's not like there are just two things they encountered when they got there or four things on the battlefield.
Here are some things, but I really think you should look at your book.
- A kind of climate they were not used to.
- People they could not usually communicate with.
On the battlefield:
- Difficulty in finding the enemy.
- Usually won battles when they could actually catch the enemy and force them to fight.
- Not very good help from the South Vietnamese who were our allies.
- Booby traps