One nation shall be mightier than the other (Genesis 25:23).
These words were part of God's explanation to Rebecca, when she asked why her pregnancy was so difficult. God told her that the two children she carried, Jacob and Esau, were struggling within her, and prophesied that this struggle would be an eternal one. At some points in history, Jacob would triumph; at other times, Esau would triumph.
The Tanya (Chapter 9) states that this struggle is not only between the Jewish nation and its adversaries, but that it also exists within each individual. Within each person are a Jacob and an Esau - a Divine soul which strives for sanctity and an animal soul which strives for physical gratifications. Like wrestlers, one may have the superiority at one time, and the other at others.
I strongly disagree with the author of those books which assert that people can attain inner peace and be free of struggle in life. Those who do not attain this desired tranquility therefore feel deprived. They may try many ways - even alcohol or drugs - to attain this assumed freedom from internal strife and tension.
The truth, however, is that inner peace is not even supposed to occur during our earthly existence. Our lives are an eternal struggle between opposing forces. Like opponents in a boxing match, we may get only a brief respite between rounds, only to come out fighting again.