Ulfeldt was the most striking personality at the Danish court in all superficial accomplishments, but his character was marked by ambition, avarice and absolute lack of honor or conscience.

He was largely responsible for the disasters of the Swedish war of 1643-45, and when the Treaty of Brömsebro was signed there was a violent scene between him and the King, though Ulfeldt's resignation was not accepted.

[2][3] In December 1646 he was sent as ambassador extraordinary to the Hague, but the results of his embassy by no means corresponded to its costliness, and when he returned to Denmark in July 1647 he found the king profoundly irritated.

As Steward of the Realm he was the virtual ruler of Denmark during the two months which elapsed between the death of Christian IV and the election of Frederick III (6 July 1648); but the new king was by no means disposed to tolerate the outrageous usurpations of Ulfeldt and his wife, and this antagonism was still further complicated by allegations of a plot (ultimately proven to be false, but believed at the time to be true) on the part of Dina Vinhofvers, a former mistress of Ulfeldt, to poison the royal family.

During Charles X's march across the Belts, Ulfeldt persuaded the commandant of Nakskov (a strategically crucial fortress) to surrender to the Swedish king's forces, and did his best to convince his countrymen that resistance was useless.

The fallen magnate henceforth dreamed of nothing but revenge, and in the course of 1662, during his residence at Bruges, he offered the Danish-Norwegian crown to the Frederick William I, Elector of Brandenburg, proposing to raise a rebellion in Denmark for that purpose.

In addition, modern historians have been liable to view him as a highly mentally unstable man whose lust for power ended in megalomania and insanity.