I haven't read the source material, but it's pretty clear that the unnamed enemy is the People's Liberation Army.

This movie has what could have been interesting subjects: The threat of China and the militarization of Japan. But it just deal with them in a very caricatural way.

It also fails as a pure entertainment movie, the action is very slow, characters are irrational. They still have a moral dilemma to fight while missiles rain on them.

Aircraft Carrier Ibuki is a manga inspired live-action film.

A fleet of warships attacks Japan and the nations self-defense force is mobilized. Ibuki is dispatched by the government to end the threat quickly and quietly. However, video of the attack is leaked on the internet causing widespread panic.

This is actually a very good movie that reflects the security concerns Japan has had in recent years, so it's actually a relevant film. As a film inspired by a comic book though, a few of the plot devices are not exactly based in reality. The name Ibuki is an homage to the WW2 heavy cruiser Ibuki that was refitted as a "flattop" aircraft carrier and the ship is run more like a WW2 Naval vessel than a modern warship. However, the film addresses several real world issues, like the post WW2 prohibition on having an active military impacting the nations ability to protect itself 70 years later and how the situation affects the peaceful nature of modern Japan.

Overall, I enjoyed this one.

This movie has ruined the whole point of the original manga.

Whether you like it or not, in reality, China is the main threat to its surrounding countries like Japan, Taiwan, Vietnam, etc.

Of course, the manga is a fiction. But it is based on the real world. Removing China from it is a very bad adaption.