一片石戰役，首先是由李自成率領的順軍，進攻位於山海關，由吳三桂率領的明軍。
戰事一開始如下：
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.
也就是說，單純順軍對戰明軍，順軍還占了一些上風。
這時清軍才介入戰事。明清聯軍在一天之內便擊敗了順軍，順軍倉皇撤回北京。
李自成並下令處決吳三桂父親及家眷共三十四人。