There is thean Japanese expression “Spartan way of education” (スパルタ式教育), which refers to thea quite severe way of educating youngsters.

For usTo most Japanese, ithis phrase sounds a bit negative,(,) because it reminds us of a kind of military training.

NOTE: The 2nd comma is optional; you can safely delete it.

Currently in Japan, a more flexible approach to education that emphasizes individuality is beingusually advocated.

LOGIC: 100% of Japanese do not agree which kind of education is best, so add the word "usually."

However, the movie “300”, a 2007 American film that depicts theone of wars between ancient Greece and Persia, gave me a glimpse into the background of this phrase.

LOGIC: There were *several* wars between ancient Greece and Persia, so this wording is better.

The film’s title, “300”, means thenarrates how a small number of Spartan soldiers who died fighting against much stopped more numerous Persian invaders, died in the process.

They were indomubitabley tough men, trained In Sparta, where most little boys were expected to undergo rigorous training to become a formidable soldiers in the future.

indubitably --> 間違いなく

The 300 of elite Spartan soldiers in the film were brave enough to face off themany enemies and struggle to ward them off, but finally they fell due to the betrayal of a Spartan, who wanted to fight with the 300 men but was denied entry because of his physical disability.

NOTE: Most historical accounts differ from the movie. (A local person who wanted money betrayed the Spartans.)

I was really shocked by this factstory.

NOTE: What was in the movie was probably not a "fact." (Many Hollywood movies are lax with historical facts.)

There is thea Japanese expression “Spartan way of education”(スパルタ式教育), which refers to thea quite severe way of educating.

The "The" in "THE Japanese expression" or "THE severe way" makes it sound like there is only one expression or way. Unless it was already clear which expression was being discussed, I would recommend "a" or one could more casually say "So, there's this Japanese expression..."

Currently in Japan, a flexible education that emphasizes individuality is being advocated for.

One usually advocates "for" something. I don't advocate a new education system, I advocate FOR a new education system!

The 300 of elite Spartan soldiers in the film were brave enough to face off with the enemies and struggle to ward them off, but finally they fell due to the betrayal of a Spartan, who wanted to fight with the 300 men, but was denied entry because of his physical disability.

He was resentful of this and instead provided useful information useful to the enemy.

"provided information useful to the enemy" can be inferred, but makes it feel confusing who received the information.

I think the Spartan combat training was essential to defending their country and loved ones, however, the director may be trying to say that it can also be harmful, if taken to an excess.

There is a Japanese expression “Spartan way of education” (スパルタ式教育), which refers to the quite severe way of educatingeducation system.

Severe sounds a bit more unnatural here, and strict or rigid might be a better choice.

For us Japanese, it sounds a bit negative, because it reminds us of a kind of military training.

Currently in Japan, a more flexible education that emphasizes individuality is being advocated for.

However, the movie “300”, a 2007 American film that depicts the war between ancient Greece and Persia, gave me a glimpse into the context of this phrase.

"the context of" adds an indirectness to your statement. Instead of drawing a direct connection between the movie and スパルた式教育, it shows that you gained insight about the origins of the phrase and its nuances.

I hadn’t heard of this film until recently, when I came across the history of a war in BC 480 called “The Battle of Thermopylae” in B.C.480 while reading a textbook on ancient Greece and decided to watch it.

The film’s title, “300”, means the numberrefers to the size of the group of Spartan soldiers who died fighting against much more numerous Persian invaders.

Adding this phrasing clarifies that the size of the group was 300, and that they all died, as opposed to a group of unclear size, of whom 300 died.

They were indomitabley tough men, trained Iin Sparta, where most little boys were expected to undergo rigorous training to become a formidable soldier in the future.

The 300 of elite Spartan soldiers in the film were brave enough to face off against the enemies and struggle to wardrepel them off, but finally they fell due to the betrayal of a Spartan, who had wanted to fight with the 300 men but was denied entry because of his physical disability.

Stylistic choice, but "ward off" sounds more like scaring them away than successfully defending against an attack.

He was resentful of this and instead provided useful information useful to the enemy.

The traitor narrowly escaped from the Sparta’sns' abhorrent practice of abandoning weak or disabled babies.

I think the Spartan combat training was essential to defending their country and loved ones, however, the director may be trying to say that it can also be harmful, Iif taken to excess.