Back in the 90's when I was in school and the US imposed sanctions on India for its nuclear tests, we cried hypocrisy. How could the US punish India for building nuclear weapons when the US also stockpiles them? I've come a long way since then, but only after a lot of self-education, education which I unfortunately didn't receive in school.
Self interest and game theory
How countries evolved
But what if there's a change of leadership and policy? Now one civilisation grows stronger, and begins to conquer more land and people. The other civilisation has a choice, do they ignore the first one, join forces with it, let themselves be conquered and assimilated, or turn into conquerers themselves to avoid being taken over? Also, military might isn't the only tool to use in your defence. There's also religion, which makes cultural assimilation easier, and economic conquest, where countries subjugate each other economically. Trusting your neighbour explicitly means giving them a chance to exploit you, and you won't do that if you think there's a chance that they will. Replay this scene for different groups across thousands of years, and through numerous conflict and death, we have the world today, fragmented groups with different value systems and ideals, terrified of being exploited or losing out. Hence, there's not much trust.
Self interest and trust
Which is not to say that trust doesn't exist. It does, but it comes about when it's in the players' self interest. The US and Canada can have a porous border because there's very little risk of a war breaking out between them because they have been at peace for so long and have reached numerous lucrative trade agreements in the process. If this peace was broken by say, America invading Canada and taking even some of their land, it would hurt trade, and Canada might align itself with an enemy of USA. It pays to keep your neighbours as allies to act as buffers between your other enemies. What we call international trust is ultimately all about money and security. Self-interest wins.
Here's another example - the Nordic countries being at peace. Denmark, Finland, Sweden and Norway all have porous borders, as do other European countries. The likelihood of a country misusing this trust and upsetting the status quo is low because the consequences would be dire. Border security would be strengthened, relations would sour, money would be lost, everyone would suffer. The payoffs from committing such an action wouldn't justify the costs. This is why the Nordic countries are at peace. They weren't always at peace. A long time ago they were at each others throats. But back then, it paid to conquer and kill each other more than it did to trade and cooperate freely. This also explains why Russia recently annexed part of Ukraine. The payoff (in terms of access to resources and trade routes) exceeded the cost (meagre threats from NATO?). Self-interests wins.
Look at the two major power blocks today - the US and China. It used to be the US and the USSR. Preceding World War 2, the US was just another country gaining affluence through trade and innovation. Following World War 2, it emerged as a dominant superpower and its alliances with a number of European partners was sealed. But competition emerged with the USSR, whose economic and social polices rivalled that of the US. Here we have a case where countries have an internal value system linked to their economic systems, so they become economic competitors to protect their social systems. The US was terrified of communism, and the USSR was intent on spreading it. So they both embarked on policies of expansion. The USSR annexed and funded countries that embraced communism, crushing any opposition. The US did the same, backing fascist murderous dictators worldwide as long as they rejected communism. The devastation this wrought was immense in terms of human life. But it was in both countries' interests not to stop their activities, because then the enemy would have an upper hand.
You also see this mirrored today with nuclear weapons. No world leader truly believes that these weapons are good, but they can't help keeping an arsenal as long as their enemies have them too. It's only the smaller countries with no ambitions of power that don't need nuclear weapons, but they are either aligned to a power block (like Bhutan), or are not threatened by one (like Oman).
Attaining world peace
One way to do this is to build trust between all the different countries that currently exist. One way to do this is by removing any perceived threat between two countries and increasing trade opportunities. And you do this by economically developing every country equally. Remove economic gaps, invest in education and healthcare, make all countries economically powerful so they can serve as trading partners with each other. This serves as a status quo, a deterrent to attacking each other. Over time, this becomes trust.
Of course, this should work better if the countries have similar value systems, as differing value systems pose a threat. For example, the world's number one economy - China - is communist, while the US-NATO power block isn't. Both blocks trade profitably with each other, but mistrust exists. A common value system would probably remove this. Remember that power blocks are only formed as reactions against perceived threats from other blocks. The other way to attain world peace is of course to ensure that there is only one power block in existence - yours.
This Crash Course World History series is extremely informative w.r.t observing patterns in group behaviour across human history. By watching a concise approximation of human cooperation and conflict across time, you begin to observe patterns in group behaviour. Watch if you have 20 hours to spare.
World history Part 1 - http://www.youtube.com/playlist?list=PLBDA2E52FB1EF80C9
World history Part 2 - http://www.youtube.com/playlist?list=PL8dPuuaLjXtNjasccl-WajpONGX3zoY4M