The reason because the Sarajevo assassination escalated into WWI is because of the insane network of alliances in Europe at the time. Serbia was allied with Russia, so the latter declared war on Austria-Hungary, who was allied with Germany, which attacked France and Belgium, which were allied to Great Britain... you get the idea. Now, the landscape is more fragmented, with lots of small powers changing sides constantly, lots of irregular groups, etc; and even if a war is triggered, fear of said war escalating usually limits the conflict to a particular region with the various powers supporting one side (e.g: Syria). Even if there is a network of alliances, the sheer amount of people involved and the fact that nuclear weapons exist keeps any crisis from escalating into world war.

I agree that far-right ultranationalists (and yes, Trump seems to fall in this category) and radical islamists are the biggest danger. The present-day climate of fear and islamophobia perpetuated by the far-right only serves the islamists purpose, to convince Muslims that the West is against islam and that they should all fight against the "crusaders", while the far-right seems to think that hate and endless war against anyone who isn't exactly like them is the solution to every problem. North Korea is a psychotic dictatorship, but Kim Jong-Un knows that if he ever uses his nuclear weapons they will get destroyed, while Russia is a concern in the sense that they would benefit from a divided, nationalistic Europe (and is actively attempting to achieve such a goal) as that would allow them to bully their neighbors into submission.

Also, I don't think we should use the term "terrorism" so loosely. A terrorist is somebody who uses violence to intimidate or coerce, usually for political purposes. Hitler, Mussolini, etc, were not terrorists per se; what they did falls under the term "state violence", if they committed such acts at all. The millions who died under Mao's rule in China, for example, were mostly due to famines caused by his disastrous economic policies.