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