If each country one by one started to follow Democratic Peace Theory than we can slowly help promote world peace without it being harsh and fast change. It will be a smooth switch over that allows room for disagreements, but discourages threats and the use of military to try and get their way.
Yes, they should try to follow the Democratic Peace Theory. If war were decided by the people who had to fund the war and fight the war, it would possibly occur much less frequently. It is unfair and makes no sense that those who decide if a country will go to war are never the people who must fight the war or pay for it.