In November 1643 Lieutenant-Colonel John Moore, the parliamentary governor of Liverpool, launched a military expedition across the Mersey, intended, among other objectives, to silence a royalist battery at Birket garrison in the Woodside district of present-day Birkenhead that had been bombarding the town across the river. The operation was carried out in the daily expectation of the arrival, in the Mersey or Dee area, of royalist reinforcements from Ireland, threatening not only Liverpool but parliament’s control of the North West. Moore’s venture, undertaken with limited resources in the immediate aftermath of a withdrawal of Lancashire forces from Cheshire, is an indication of how seriously this threat was taken by the town. In the event, the effectiveness of the operation seems to have been slight, or only temporary, but at the time it was believed by royalists to endanger the incoming Irish forces, a belief that probably explains the first wave’s inconvenient disembarcation in Flintshire instead of Neston in Wirral. The present article examines the political and military background to these events and recounts the setting up of the Birket garrison. It also considers the details of Moore’s expedition in the light of the testimony of local persons collected during the post-war sequestration process and, not least, the misapprehensions arising on either side.