Sitting down with an enemy over a cup of coffee is a great way to bring people together and to help resolve their differnces. It could teach them that they have more in common than they think and help them to reach compromises and solve problems. Ti would take a lot of work.
Look at wars in the middle east over religious differences. People fight to the death against people of other religions because of their zeal that their religion is "right". Similarly, look at American politics, like people at a Trump rally. No matter the obvious lies that come out of Trump's mouth (even when it's on video disproving what he is saying), people applaud what he has to say and fervently support him, just because the opponent is hated.
Since civilizations began, there has always been war. One nation invading another to acquire resources and land is still ongoing. It is naive to think that any leader bound on domination would suddenly become willing to accept peace for a 'cup of joe.' While many citizens of the world would love for this to happen, the obvious nature of man to war with one another cannot be overcome this way
While coffee tends to make everyone I know much happier, I do not think it is going to achieve world peace. With the rise of ISIS and even with the turmoil on our own US soil, the world is not a very happy place. It is going to take much more than coffee to fix these problems.