King John Casimir II Vasa ('Jan II Kazimierz Waza' in Polish) of
Poland fought in the Thirty Years' War (1618-1648) and was a Jesuite
Brother, before ascending to the Crown (after the death of his
brother, Ladislaus IV) in Jan. 17, 1649.  According to a source of low
reliability, before becoming King he became a Cardinal in the Church
and served in Rome for some time.  Please verify, debunk and/or add to
my knowledge about the early, pre-reigning period in his life.

Dear doctoray,

Here is what I have found out about John II Casimir's life before he
became king of Poland:

In 1635, John Casimir offered to lead a combined Habsburg-Vasa army
based in Silesia. However, Emperor Ferdinand treated this obvious
Polish attempt to establish firm Vasa-Polish influence in Silesia with
deep suspicion. Instead of forming the proposed army, Ferdinand only
appointed John Casimir colonel and gave him command of a Habsburg
cavalry regiment in August 1635.

John Casimir was promised a Spanish pension in 1636 and received the
Golden Fleece. In 1638, he sailed for Spain to take up an offer of an
admiralship in the Spanish navy and the viceroyalty of Portugal.
However, he unwisely decided to land near the French mediterranean
port of Marseilles, which led to his arrest by the French and two
years in captivity. So he never arrived in Spain to take up the
commission.

In 1640, the French released John Casimir. He entered the Society of
Jesus (Jesuit Order) in 1643. He was created cardinal priest in the
consistory of 28 May 1646, but he never received the red hat and the
title. In a letter to the pope written in Warsaw and dated on 9
November 1647, resigned the cardinalate. In the consistory of 6 July
1648, the resignation was presented to the Pope, who accepted it.

John Casimir then married the widow of his brother, King Ladislaus IV,
who had died without a male heir on 16 May 1648. On 20 November 1648,
he was elected King of Poland, and crowned on 17 January 1649.



Sources:

"1648 - War and Peace in Europe. Essay Volume I: Politics, Religion,
Law and Society". Published by Veranstaltungsgesellschaft 350
Westfälischer Friede, 1998. ISBN 3-88789-128-7

The Cardinals of the Holy Roman Church: Consistory of May 28, 1646
(III) - WAZY, S.J., Jan Kazimierz (1609-1672)
http://www.fiu.edu/~mirandas/bios1646.htm

Department of History, University of Wisconsin-Madison: Poland in the
Seventeenth Century
http://history.wisc.edu/sommerville/351/351-104.htm

Wltchronik: Johann II. Kasimir
http://www.weltchronik.de/bio/cethegus/j/johann2k.html

Polska-Info: 1609-1672 - Johann II. Kasimir
http://polska-info.de/de/Sections+index-req-viewarticle-artid-21-page-1.html

Search terms used:
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"john ii casimir" 1609 jesuit
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"jean ii" pologne 1609 1647 cardinal
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"jean casimir" 1609 1648
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"casimir V" 1609
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"john casimir" 1609 cardinal 1648
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Hope this is what you were looking for!
Regards,
Scriptor