It is an encouraging attempt! .
1-Do mention the word limit while posting Precis. The precis should be 1/3 of the original passage.Therefore,i feel that this one is under length.
2-Try to use transition words for better connectivity and coherence.
3-2nd paragraph is fine
4- i have tried to rewrite first paragraph. Feedback is welcome.

War has been the cause of break down of the human civilizations since prehistoric times.However,In the early civilizations,the monetary and military benefits of the wars outsmarted the devastations occurred and the defeated were seldom crushed to extinction.But, It is when the aggressors advanced their military equipment and developed powerful disciplined armies, the full scale of destructions of wars became clear, capable of extinguishing entire civilizations.