【是这样】：
说话人为了强调、突出Unsurprising这个词,使之更为醒目,前面省略了“It is”.完整的应该是：
“It is unsurprising then that China should have a long history of mucking about in Korean politics,a history which has often brought it into conflict with that other great Eastern power,Japan.”
“a history which has often brought it into conflict with that other great Eastern power,Japan.”是a long history的同位语.“which has often brought it……”为a history的定语从句.中间的it是指前句中的“China”.
【整句话的意思是】：
“因此,这绝没有什么好奇怪的：中国在朝鲜政治（事务）上纠缠悱恻,原就有着悠久的历史,一部常常使之与另一东方列强——日本,陷入冲突之中的历史.”