The Cossack revolt shook the Commonwealth to its foundations. Disastrous defeats at Zhovti Vody (11-16 May 1648) and Korsun (26 May), where the Polish grand hetman Mikolaj Potocki and field hetman Marcin Kalinowski were captured, were followed by the humiliating rout at Pyliavtsi (23 September), when the levée-enmasse once more demonstrated its worthlessness by simply running away. The death of Wladyslaw IV on 20 May, by plunging the Commonwealth into the uncertainties of an interregnum, greatly hindered the political and military response. It was not until 17 November that John Casimir was chosen to succeed him.

The election of John Casimir, born in 1609 to Sigismund Ill's second wife Constance of Austria, was something of a surprise, for he had not started as favourite despite being the elder of Wladyslaw's surviving brothers. He had done little so far to endear himself to the nobility. As a younger son of an impoverished dynasty which lacked an independent financial base after Sigismund's deposition in Sweden, John Casimir's political and financial position had been uncertain. Such resources as the family possessed were devoted to maintaining Wladyslaw in the style befitting the heir presumptive.