The point made in this book is interesting: that the impact of the battle of Marathon is underappreciated, and it's not widely understood that this was solely an Athenian military triumph that was accomplished without help from the Spartans. The authors states that Spartans get too much credit for military achievements in general, whereas the Athenians have a lot to be proud of in that arena. The author does a good job of explaining his premise and then stepping back in time to set up the situation that led to Marathon, then how it happened, and then a quick summation of why it mattered so much. All that structure is well and good in a general history.
However, the book isn't well-written, and while it's clear enough on most things, it often falls into boring lists of city-states, civic leaders or other stuff, none of which would mean anything to a general reader. In that sense,it falls into the trap of too specific for a general reader and not enough detail for someone with some prior knowledge. But it's meant to be a first book, not a comprehensive review.
The story goes something like this. Greece into about the 7th century BC was a series of small city-states that were constantly at war with each other. Some were more successful than others, but even the largest (like Sparta) had to ally themselves with a lot of other cities for defensive purposes. Along comes Athens, and the century of the 500's BC it develops a unique system of governance, combined with military excellence, that becomes the standard-bearer with which we are familiar. Rule of law, democracy, the start of drama and memorable sculpture and architecture, etc.
But within 15 years of this happening, the biggest empire the world had ever seen (up until that time), Persia, decides it's had enough of the upstart Greeks, and it sends an invasion force in 490 BC. Had Persia succeeded, all the stuff that Western civilization got from Greece probably wouldn't have happened, and it's "experiment" in participatory democracy would have been seen as inadequate to fighting to defend turf.
But Persia is defeated by Greek city-states, actually 3 times. The first time is a small-bore action on the edges of Greece, and the Athenians help to some degree with ships. The second time, Persia comes with a bigger force, and really goes after the leading cities and key islands in the Aegean. In this campaign, all goes well for Persia until Marathon. There on a wide plain 22-26 miles from Athens (depending on the path you take), the Athenians with a little help from one ally city-state (but no help from Sparta), defeat the feared Persian force. Then, on the same day, those soldiers march back to Athens with 60 lbs of weapons, shields and gear per person, and then arrive in time to stare down the Persian naval fleet that has arrived to take the city by sea while the soldiers were engaged at Marathon. The Persians sail away.
In the third effort, they return 20 years later and have their battles with Sparta, and those are better remembered, which is why we credit Sparta with defeating the Persians, rather than the Athenians.
In describing those battles (all three since the tactics were similar), the author explains how the Greeks used a different military style than did the Persians. The Greek "hoplite" army was heavily armored with heavy wood and copper round shields, copper helmets and shin guards. They were basically impregnable from the front, and when they got close enough they spiked people with heavy spears and swords. They could be beaten if you got on their flanks. The Persians carried small wicker shields and no real protection on their bodies, and they fought primary with arrows. They moved swiftly and coordinated with cavalry charges to create confusion in their opponents, and then charged in with small swords for the final kill. Power vs. speed.
At Marathon, however, the Greeks added a surprise. They moved across the plain as the aggressor, even though they had a great defensive position. When they reached about 150-200 yards from the enemy, they started to run, thus minimizing the time that arrows could hit them. And they smashed the lighter force with their power.
One of the best things about this book is how the author explains how all of the above was developed, and what was innovative at the time, and also what is known and what is legend. Example: Legend is that the Greeks ran nearly a mile across the fighting area to engage the Persians. Nonsense, says this author. Since they knew the Persian arrows didn't go much more than 150 yards, they wouldn't have started running until that point.
The other thing that this author does is explain how Athens' unique governance created an environment in which the brave decision to fight the Persians was taken. Since all citizens (that is, males who weren't slaves) had a voice in the government, a genuine voice, those citizens really did feel they had something to defend. This was a new idea in world culture, and it explained why a huge percentage of the eligible men came out to Marathon to do battle -- the author estimates something like 75% of the men who could afford the military equipment. Not only is it a contrast with the Persians, who had their core people as well conscripts from subjugated peoples, but it was different than Sparta, which had an elite warrior class that was built on required military service, but which always feared an internal uprising from oppressed peoples within Sparta's territory.