The parshah speaks of the many battles that klal Yisrael fought. As they traveled through the desert, they were confronted by their enemies, time and time again. Tired, thirsty, exhausted from the weary journey through the desert, they had just been attacked and beaten by an implacable foe, Amaleik. They cried out to Hashem – they made a vow to the Lord – and He came to their rescue. Although the Canaanim were powerful, Hashem said, “Why should I trouble My children to besiege every city?” He gave all the warriors the idea to leave their cities, and they gathered in one place, where they were slain (Rashi).