Various theories have been suggested in this context
elaborate this point, once you've informed the reader that theories exist you must then at least list those theories.

Every nation at the height of glory wishes to enslave the smaller states.
Examples please! This is the part where you can show off your knowledge don't miss the chance.

Search out pages of history (incorrect grammar. It should be Flip through the pages) and you will find many instances proving the above quote (Insert "Correct" here, a thing can be disproved also, readers appreciate a clear and specific argument). And again examples please, we'll flip the pages of history for ourselves later but for now since we're reading you essay we want know what you have to say about the topic. Then why we always fail to stop wars?(incorrect grammar again, to be grammatically correct the sentence would include "do" after why. But the content of the sentence itself is questionable have "we" always failed to stop wars? (Also specify who is "we"?) Wars have always been stopped, your topic is preventing war not stopping it. You stop something once it has begun. When you prevent something it means it will never start in the first place.

Your main thesis regarding prevention of war "A mass demonstration on peaceful grounds on largest possible scale" is interestingly idealistic. Also you're limiting the content of your essay by proposing only one method rather than exploring varying methods throughout the essay. If you have strong enough arguments to support your current thesis by all means go for it but present is it as the best solution after detailing other methods.

In summary be specific when presenting an argument, write clearly (give the reader as much information as possible), Make sure you know what a word means before you use it, give examples to support your statements and lastly when you're done writing, read what you have written pretending someone else has presented their piece of writing to you for feedback.

Hope that helps. Sorry for the detailed post, I'm inclined to help over zealously when I detect potential.