Complex sentence: In March 1999, the JCG tried to intercept vessels of unknown nationality suspected to be North Korean, but the unknown vessels were too fast, and the Japan Maritime Self-Defense Force had to be called to intercept the unknown vessels.
Split into simpler sentences: In March 1999, the JCG tried to intercept vessels of unknown nationality suspected to be North Korean, but the unknown vessels were too fast. The Japan Maritime Self-Defense Force had to be called to intercept the unknown vessels.