To bring something like an anti war legislation into effect would need to change quite a few public sentiments first: The hero worship, battle hymns, and rewarding the most effective killers with medals have to go; do away with Remembrance Day and Dawn Services; expose the illusions of a just war, the illusion that with military might you can have an effective deterrant, the belief that you have to defend your country by force.
Let the law against killing people be an absolute, overriding all other conditions: Anyone killing another person has to stand trial for murder as a war criminal. Let each one carry the responsibility on his / her own.

Once you have these conditions accepted by the public, world-wide, then you might have a chance with your international peace agreements. Without this you will most likely have to wait another 76 years for "Peace in our Time"

Consider the people of North Korea, and the disastrous tyranny under which they are suffering -- in North Korea, every hour is Earth Hour! Banning war would mean that other nations were unable to help them, or others like them. Taken to its logical extreme it would mean that any attempt to violently resist the actions of a government -- by protestors, by freedom fighters, by rebel groups like those which brought down the governments of South Africa and Libya -- could be regarded as an act of war, and punished accordingly by international fiat. Defying your own government when it's trying to kill you is bad enough; defying the whole world would be far worse.

Let's be aware of the horrors of war, and avoid glorifying it in any way. But let's also be aware that there are worse things than war, and keep it in our armoury as a weapon of last resort. There are many dictators and would-be dictators who would rub their hands together with glee if they thought there was a real possibility that the international community would renounce war.

War, analogous to inflation, is often the result of too many people chasing too few resources. Therefore, meeting the challenge of uncontrolled population growth and supporting a more equitable distribution of resources are measures against war. Planned parenthood, educating girls and public health measures reducing infant mortality relieving pressure on families to have many children are some of the measures that can be taken against uncontrolled population growth. Separation of church and state ensuring that the power of the state cannot be used to promulgate superstition will help to promote clear thinking making it more difficult to push killing for 'God and country.' The arms trade except for clear instances of defense must be curbed. Corporate exploitation which increases the difference between rich and poor also is a factor in increasing social tensions which lead to war. North Korea has a horrible dictatorship. However, the horrors of a war to overthrow that dictatorship may not be necessary. No country can be completely isolated from the outside, and North Koreans becoming more and more aware of the fact that there is a better world outside can be given aid to free themselves. Let's reclaim the Kellogg-Briand Pact and take steps to support it.

Two things I would add:

Firstly, stop believing propaganda. In fact, the more a foreign regime is uniformly demonised in the world media, the more suspect we should become about why those who wield global power, so badly want this particular regime overthrown (especially when so many far worse regimes are either ignored or remain our official friends).

Secondly, one of the main reasons for continued war is the cult of masculinity, which embeds violence with masculine honour - from the playground right through to international relations. Until we finally acknowledge the global destructiveness of masculine violence, and stop treating it as the great taboo, we can expect many, many more wars to come.

A major cause of War is resource competition, natural, economic, inter-personal and whatever. Having enough to eat, be clothed, health and such basics removes a lot of the impetus. Unfortunately the hysterical, well paid Warmists, Lefties and other haters would have us (but surely not themselves) in subsistence to starving and hence are ipso facto pro War.