Four of Kagan's five chapters are briefly discussed here: the Peloponnesian War, the First World War, the Second World War, and the Cuban Missile Crisis. Kagan's chapter on Hannibal's War doesn't interest me.
Kagan is a professor of ancient history. He writes that war erupted between Athens and Sparta because Athens was not strong enough to scare Sparta into a reluctance to go to war. Kagan: "The Athenians simply did not have enough manpower to offer a credible offensive threat." Of the Spartans he writes: "Only the challenge of a superior army could have held them back, and the Athenians had no way to produce that." Putting some complexities aside, this is to say that Sparta started the war and Athens had no choice but to defend itself. Those of us who believe in nations defending themselves against aggression might ask whether there was anything that Athens could have done to avoid that great tragedy - other than scaring Sparta with an obviously superior military capability.
The Athenian journalist Thucydides found blame for the war in part at least with the Athenians. He thought they had been too inflexible in foreign affairs and too reluctant to admit mistakes. Kagan, in my opinion, is too easy on the Athenians. In the decades before the Peloponnesian war, Athens did well in forming an alliance to combat Persian aggression. Athens persisted in its war against Persia longer than it needed to, and it was coercive in maintaining its alliance, also called an empire. Athens at some point would have been better off had it let go, had it held back militarily - a lesson Japan and Germany failed to follow in the 1930s. If Athens had accepted a more modest standing among the other Greek states it would have made itself less feared. It is difficult to claim that Athens provoked the Great Peloponnesian war, but it is not so difficult to go beyond Kagan and claim that Athens would have been better off had it maintained a more equal and cooperative standing with its fellow Greeks. Kagan differs from those Athenian aristocrats who disliked the zeal and harshness with which their city extended its power. Rather than finding fault with Athens, he sums up his chapter on the Peloponnesian War with detachment:
Pericles and the majority of Athenians, however, had great confidence in the promise of the new, untried, strategy that their navy, walls, and empire allowed them alone to pursue. They counted on it to deter their enemies from fighting and, when that failed, to bring victory. So the war came.
To repeat my opinion expressed elsewhere on this site, Athenians went to war because they and others in their day believed in empire, because people feared being dominated by others and because the Athenians did not fear war enough that they would try harder to avoid it. The war was to be a lesson in the benefits of caution over optimism and self-confidence, in the benefits of restraint and modesty in pursuing one's interests and the benefits of cooperation and compromise.
The war was a great tragedy, as Kagan points out, especially for Athens, which lost.
On the origins of the First World War, Kagan finds fault with Britain, writing:
Suppose they [the British] had faced the fact that only the assurance of a large, well-trained British army that could quickly come to France's aid in case of attack could make a German victory in the West impossible and obviously so.
Here again is Kagan's belief in scaring a potential enemy into a reluctance to make war, and here the question arises as to its relevance. Austria's hold on Bosnia-Herzegovina created the spark that ignited the First World War, with Russia deciding to support their fellow Orthodox Christians in Serbia, whom the old Habsburg monarch, Franz Joseph, chose to war against. The German nation went to war believing it was defending itself against invasion by the Russians - something an old neo-conservative like Kagan might understand. France and Britain need not have been a part of the war. Germany's monarch was delighted to learn, falsely, that France would stay out of it, but France went to war fearing for its power if it did stand with its alliance with Russia. Britain might have prevented Germany's involvement in the war had it let the Germans know earlier than it did that it would join the war with France, but that is a mere "what if" that speaks little to the actual origins of the war. The cause of the First World War was Hapsburg imperialism mixed with Russian shortsightedness and German blunders. Kagan provides a lot of detail in telling a long story but makes no such summary.
Kagan does better with the origins of the Second World War. He faults Britain again for its military readiness, but the United States and France could be included in this for not maintaining the military alliance they had during the First World War and to have appeared ready to use it defensively against German aggression. Kagan is correct in arguing that Hitler launched his aggressions believing in the reluctance for war of his potential enemies. But this is not to say that one must be careful in applying this as a lesson - as did Kagan in 2003.
Finally, the Cuban Missile Crisis of 1962. The Soviet Union had an alliance with Cuba, and Chairman Khrushchev was afraid that the U.S. might again send an invasion force against Cuba. This should not be ignored while acknowledging Kagan's observation that Khrushchev had been impressed by Kennedy's failure to use U.S. military might to back up the Bay of Pigs invasion in 1961. Kagan suggests that President Kennedy should have ordered a greater U.S. participation in the invasion of Cuba. He writes:
The Cuban missile crisis demonstrated that it is not enough for the state that wishes to maintain peace and the status quo to have superior power. The [Cuban missile] crisis came because the more powerful state [the U.S.] also had a leader who failed to convince his opponent of his will to use its power for that purpose.