Well if you can agree to get everybody in he world to stop fighting, then great, we won't need weapons. Looking at the US as an outsider I can see that as bad as spending billions on the military is, you guys need to do it. You simply could not depend on all the other countries to not invade. Even if all countries of the world were magically disarmed overnight, human being instinctively want more power and someone will want to take over somewhere else by force.
Besides that, military spending has helped to invent and develop many aspects of science and technology. Rockets, airplanes, GPS, the internet, computers, communications etc. all benefited from military spending.
I do think the world will get less militant but it won't get noticeably better in our lifetime. If we look to the developed (basically the northern hemisphere), we all get along now. Communications and such make war much less likely between two fully developed nations and even though the likes of say America and Russia might still seem like rivals, a war would be completely absurd now. Propaganda couldn't happen again the way it did during WW2 to turn an entire nation against a neighbouring one. Whether you like the EU or not, you have to admit that it provides a lot of security for Europe, like there had never been before.
No I would say once the rest of the world manages to catch up in terms of things like education, health, technology and infrastructure, the world will start getting along better and war will become less frequent. They will catch up too. The likes of Brazil and China might seem poor now but they're already most of the way there and other developing countries will follow.