Contrary to the wishes of his own sovereign, Sigismund I, whose pro-Austrian policy he detested, Laski entered the service of Janos Zápolya, the Magyar competitor for the Hungarian throne, thereby seriously compromising Poland both with the Emperor of the Holy Roman Empire and with the Pope.

[2] France granted Zápolya 20,000 in gold, five thousand of which was to be forwarded immediately, but on Laski's return, he found his patron a refugee in Transylvania, whither he had retired after his defeat by Ferdinand I in the Battle of Tarcal in 1527.

On being released by the interposition of the Polish grand hetman, Jan Tarnowski, Laski became a violent opponent of Zapolya, and openly changed his allegiance, allying himself with Ferdinand I.

In 1539 Ferdinand sent Lasky to Constantinople, to denounce the existence and details of the secret treaty of Nagyvárad (1538), to prevent Suleiman from helping Zapolya against a planned Habsburg offensive.

[2] He remained a prisoner in Belgrade for some months, accused of the murder of Suleiman's supporter, Antoine de Rincon, ambassador of the King of France to the Sultan between 1532 and 1541).