How did Vietnam’s physical geography pose an early challenge to the U. S. military? The weapons of the U. S. were useless in Vietnam’s farmland. The weapons of the U. S. were useless in Vietnam’s farmland. The enemy could move during the monsoon season while U. S. soldiers could not. The enemy could move during the monsoon season while U. S. soldiers could not. Vietnam’s jungle terrain made it difficult to find the enemy. Vietnam’s jungle terrain made it difficult to find the enemy. U. S. soldiers could not hide from the enemy in Vietnam’s lowlands. U. S. soldiers could not hide from the enemy in Vietnam’s lowlands.