Where is the evidence ?

Great answer! What else?

This is the topic for my debate class, and I am completely drawing blank.

you can start the debate by saying that the statement is too vague. In History both, love and war have conqurred the world 
love example- Budhha and  Christ but later

Christians enslaved the world with power not with love in europe there was slave trade and it was flourishing during the dark age. This means those who were calling themslevs as Christains were not conqurred by chistianity otherwise why they were tarding the slave

Buddhist kings tryied to conquer the world with war, like japan was buddhist during second world war but they killed many ppl that means budhhissm didnt conqurred the japanese king, otherwise why he wages the war?

Similarly love of Mahatma Gandhi didnt conquer anybody who is the follower of Gandhi these days? 

Christiany says love, Budhhism says love, Gandhi says love but all these failed to conquer human because we have so many wars, enemicity and hatered.
Yeah, they sound good. Love is among human from ancient time  but it failed to conquer human thtas why we have war, crime,hatered etc etc

Great answers, _______, ( I don't know what else to call you :-) ) Thank you. I will use everything you wrote. This is a great starting point.

As far as the evidence goes, the problem is that the only person that I could think of that is not conquered by love are sociopaths. Who else is not conquered by it?

dont deviate from the point stick to the point.
there are ppl who call themselves atheists, they dont follow any so called love of christ, Budhha, Gandhi but not all of them the are bad. These days there arne many ppl are  atheists. They arnt harming any one and if they have harmed so did the harbinger of gandhism, bushhism, and christianity. and example of these I gave above.

Love is always conditional and it always comes with strings attached to it. Anything that comes with strings attached to it is not enough to conquer the world. And you can use the above examples to show that all of them tried for a reason, alienating someone else. Love binds only certain group of people or divides people or you can also say love exists beacuse there is hatred just like light and dark. And hopefully you do not beleive in the above lines.

Goddamn,

I think you are right. I think I actually do believe that love conquers all. That is why it is so difficult for me to think from the other side.

I would debate against this argument - if it is one - by showing the ambiguity in the meaning of this sentence. What do you compare love with when you say it is the greatest weapon? Would you compare love with other (strong) weapons, or would you compare love with its antonyms and other feelings?

For an example, I would start by discussing the meaning and the connotation of conquering, and then proceed to explain how love is not about conquering. If the objective of some action is about conquering, then perhaps it is not love or an action guided by love. I would then say that love, as compared to it's antonym, hate, or any of the in between feelings, has a bigger and lasting impact - and perhaps cite a few historical examples. Next, I would discuss the meaning, significance and the objective of a weapon - even in its mildest form, a weapon is something used against someone. The objective of love, and the objective of a weapon, cannot be the same. To think of love as a weapon is similar to - I would bring up some satiric analogy here to show the incoherence. Now, since you sort of come close to proving that this argument is not sound, you can perhaps defeat the argument by stating exactly what the statement means in some other words.

Here is my take on that.

As a social creature, there is an intense amount of interaction that goes on in life which could involve a lot of playing different roles. For example when you are in front of your boss you act different and similar is the case when you are with your girlfriend or a teacher. These immense amount of interaction could lead into a psychological frustration and you'd feel that there is nobody on your side as you never have the chance to be yourself. This will now be manifested into a form of quest for something which you wouldn't know resulting in a troubled soul and a feeling of lack of accomplishment. Now, deep down you start to get dissatisfied and it could bifurcate into two seperate but distinct path. Either you get motivated and work hard, or you resign. If you get motivated and work hard, then with time you could still acquire your desirables but will still lead you into wandering coz by then you would still be dissatisfied despite the fact that you've gottan everything you desired as your soul still feels empty. On the other hand, if you've resigned except for whining and crying there is nothing else you could do so you would be regretful throughout your life. Both these sense of lacking is only overturned when there is someone or something that you could associate with your life, that helps to be yourself, and also that could allow you to dissociate with everything else if you chose to do so and still be happy. And that is love. In other words it could be your psychological win or a psychological surrender but will still allow you to be yourself and be happy. After all according to Mr. Aristotle our end goal is happiness. Think about the song by Beatles "bla bla bla bla bla bla bla... money cant buy me love" or even the concept of yin and yang.....
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"Love is the greatest weapon because, in the end, only it conquers everyone."

By definition weapon is something used to injure, defeat or destroy.
So by the definition of weapon itself, the argument stands defeated. Love is not a weapon. If someone uses love as a weapon then it is not love. Anything used as a weapon may conquer physically due to fear, but using weapon you cannot conquer others from their heart.

If love did what the argument said, us wouldnt be fighting two wars using weapons

You could cite John Maynard Keynes "In the long run we're all dead."

Love may conquer everything, but for the goal to reach its fruition it could take decades and we only have so much to live. Therefore, although love may conquer everything, you can argue that since life and more importantly, productive years (18-60) are short lived, we may have to resort to other means. By the time love comes through for us, we may already be dead/sick or simply indifferent.

I am copying all of your invaluable feedback. Thank you. This is really helping me.

Considering all of you are struggling with finding arguments, I am not feeling as bad. I was thinking that maybe I was stupid that I couldn't think from the other angle.

Well it says, the love wins in the end. Hopefully it won't be the end of my life....