Complex sentence: On June 6, they captured Kaifeng, the capital of Henan, and threatened to take over Zhengzhou, the junction of the arterial Pinghan and Longhai Railways, and Japanese success would have directly endangered the major cities of Wuhan and Xi'an.
Split into simpler sentences: On June 6, they captured Kaifeng, the capital of Henan, and threatened to take over Zhengzhou. Zhengzhou was the junction of the arterial Pinghan and Longhai Railways, and Japanese success would have directly endangered the major cities of Wuhan and Xi'an.