During the part when you meet the French rebels you are asked to either go left toward the Flak 88s or through the house on the right. Choose left and kill all the Nazi surrounding the Flak 88 as well as the Nazi in the ditch leading to the next gun. Now blow up the Flak 88 along with the rest of the guns. Once you destroy all the guns you will find the house you would have gone though had you chosen right. Go inside and kill the Nazi waiting for you along with the rest of the enemies in the house. You'll then fall through the burning floor and meet up with your allies.