In 1519, Cortez landed on the new world with a small army. His men were heavily out numbered. Cortez ordered the men to burn the ships. Then he said that the only way to ever see home again was to be victorious. They would either be victorious or die. His men were victorious.
Retreat is easy when you have the option. When your only choice is success, you leave no room for hesitation. You waist no effort keeping doors open. All of your effort goes into your conquest.
Are you ready to commit so heavily to your goal that you will burn or ships or are you going to court failure by keeping your escape route open?