International tensions can ratchet up with very little notice. The reality is that with the level of acrimony that is possible in international relations, coupled with the nature of modern weaponry, a World War III is a likely possibility. The question really is not whether such a conflict is likely, but rather when will it occur?
Many men have tried to take over the world in the past. Many such as Napoleon, Hitler, Mussolini and others have tried to impose their rule across the globe, and many unstable men today such as the leader of Korea still desire rule with an iron fist. Because these men have followers, it will continue to lead to wars against other nations, and it is likely only a matter of time until one of these wars escalates into a world war.
A World War III is very feasible given currently global conditions. There is growing discontent, especially among western nations with China. North Korea has been stirring trouble for years, and the more extreme Kim Jon-un's actions become the more likely he is to prompt some sort of action against North Korea. The unrest in the Middle East will continue. If any major western nations are provoked to take military action, World War III could feasibly break out.
While there are likely to be wars in the future, it is unlikely that there will be one on such a large scale again. The economies, politics and cultures of most first world countries are too interconnected. If there was a large-scale war, economies of all countries involved would be ruined due to how it takes multiple countries to manufacture even things as simple as shirts.