"Reach Stripe 1" means that at the end of a Game Turn at least one friendly Unit need to be on Stripe 1.
Does this mean that I have to reach stripe 1 with all of my soldiers or is it enough to reach stripe 1 with just 1 group, leaving the others behind?
At least one Unit need to be in Stripe 1.
I reach stripe 1 but there are still enemies on the board, is the mission still a success?
Is the mission complete the moment I reach stripe 1?
Mission end is always checked at the end of a game turn.
What happens to the enemy units left on the board?
Mission 1 is a Patrol, your duty is to scout the area, not necessarily to destroy every enemy. The rest of the Platoon is expected to follow your squad to remove the enemy forces.