As required, I will provide a lengthy answer.

In the past assassinations were often an option, though like today most had some sort of security detail. An example of which (though perhaps an extreme one) can be found in the Mongol Empire, where Genghis Khan had a private defensive force called the Kheshig. The force was sizeable, being thought to be around 10,000 plus by the time Genghis died. This is perhaps why no Khan of Khans (leader of the mogul empire) was ever assassinated.

In less militaristic nations, or in nations with a smaller security detail, leaders often were assassinated. A prominent example being in Julius Caesar, though here it was his friend, Brutus who murdered him, and it was not for a war, but rather for political gain. The religious wars in the Middle Ages also claimed many monarch's lives.

The Order of Assassins or Hashshashin was an islamic group which terrorised the islamic world for the better part of two centuries until it was eradicated by the Mongols on their way through the Mohammedan Empire. Whilst they were still around, though, their ruling was generally accepted, and though many tried to defeat them, most feared that they would be assassinated should any move be taken against the great Hashshashin.

Thus in this respect, perhaps enemy leaders could be simply assassinated, and as history has shown, it can be an effective method if the leader can actually be reached.

However, in modern times it is much harder to assassinate any leader of a major nation such as America, due to not only the massive security details and defences, but also because of the intelligence agencies which play a huge role in the protection of leaders. Obviously there are exceptions, such as the Kennedy ordeal, but such exceptions are just that.

(Speculation) In terms of ending wars it is unlikely to do so in the modern age. This is mainly due to the fact that unless the enemy country in question is a dictatorship, monarchy or any kind of totalitarian state the influence of assassinations is probably minimal. This is because new leaders can be quickly appointed. Totalitarian leaders who feel under threat of assassination are probably less likely to continue fighting. In addition, in such totalitarian states the leader is often the one powering the war. Although, in many cases, even in such nations, a new leader with similar ideals and qualities (if not worse) is quickly appointed, as seen in General Gaddafi and Libya.