The British and French feared that Russian frigates based from Petropavlovsk would act as commerce raiders during the war. As the theory goes, they would be able to resupply in San Francisco or Honolulu, then range across the Pacific, attacking British and French whaling ships and merchantmen -- even raiding Australia and New Zealand.

Grainger's The First Pacific War is a good book on this topic.

Adm. Price, the British commander of the joint French-British Force was completely wrong in his theory that the Russians would launch wide-ranging commerce raids. British intelligence about Russian intentions and dispositions was horrible during the first year of the war. In addition to misunderstanding the Russian strategy, it overlooked the fact that Okhotsk had been usurped by Aian as a mainland port.

Price then compounded his errors by committing suicide right before the battle at Petropavlovsk, which left the assault in less-capable hands, and it was bungled badly.