You bet. They will be shitting their pants, crying in every international forum, raising history and trying to make other countries stop Japan from doing so.
However, it's not as cut and dry. If Japan even makes that decision, which I highly doubt because they are content to be under the USA's shadow and trust America's judgment, it's going to take them at least 20 years to make a proper military, like that of old times.
But it will definitely shift China's attention to them, and they are going to have to include Japan as a factor in their military expansionist plans.
The fear's not just historical, but psychological as well. The way they have been brutalized by the Japanese multiple times in history, any country in their place would immediately try to ensure that it doesn't happen again.