Moses told the Jewish people, "There was not even one communal [Amorite] city that was too strong for us."
Devarim 2:36
Social unity protects a society from danger. The Amorite kings were aware of this and therefore took steps to unify their subjects against the threat of invasion by the Jewish people.
Nonetheless, a society's ability to achieve unity is limited to the extent to which its members can negate their individual egos in order to submit to a common goal. G-d expects the Jewish people to surrender themselves totally to their Divine mission -- and He therefore gave us the ability to do so. Therefore, the unity that the Amorites could achieve could not compare to that of the Jews. As a result, the Jewish people were able to overcome the united front presented by the Amorite cities.
We see here the tremendous power inherent in Jewish unity and the need to foster it to the greatest extent possible -- especially since, as we are taught by the sages of the Talmud, our present exile is a result of baseless hatred and disunity among the Jewish people.
--From Kehot's Daily Wisdom