一片石战役，首先是由李自成率领的顺军，进攻位于山海关，由吴三桂率领的明军。
战事一开始如下：
https://en.wikipedia.org/wiki/Battle_of_Shanhai_Pass
At dawn on May 27,
Wu Sangui's forces were deployed in the vanguard and were ordered to charge
the Shun army, but despite disorder in the Shun ranks, their defense line did
not yield.[22] Unable to break the rebels' line, Wu's troops suffered heavy
casualties.[22] Historian Frederic Wakeman claims that by the late afternoon,
Wu Sangui's army was on the verge of defeat when a "violent sandstorm"
started blowing on the battlefield.
也就是说，单纯顺军对战明军，顺军还占了一些上风。
这时清军才介入战事。明清联军在一天之内便击败了顺军，顺军仓皇撤回北京。
李自成并下令处决吴三桂父亲及家眷共三十四人。