Charles I trapped the Earl of Essex's Parliamentarian army around Lostwithiel in Cornwall during August and September 1644. Parliamentarian cavalry escaped, but the infantry surrendered its weapons before being allowed to march away. It was among Parliament's most serious military disasters of the war.

Lostwithiel fuelled criticism of aristocratic commanders and fragmented parliamentary strategy. Narratives of exhausted, disarmed soldiers strengthened demands for institutional reform and a more professional national army.