As one of the so-called oligarchs, he established a province in Western Transdanubia, which laid in the borderlands of Hungary with Austria, and ruled Győr, Sopron, Moson, Vas and Zala counties de facto independently of the monarchs by the 1280s.

[4] During his advanced age, Ivan's younger son John the "Wolf" was born in the 1300s; after his downfall in Hungary, he integrated into the Austrian nobility, becoming ancestor of the Bernstein (or Pernstein) family.

Several magnates and Béla's closest advisors followed her and left Hungary, including Henry Kőszegi, who handed over Kőszeg, Borostyánkő (Bernstein, Austria) and other castles along the western borders to Ottokar II.

When Henry extended his dominance in the royal council, Ivan served as ispán of Zala County – where his initial lands mostly laid – from September 1272 to March 1273 (with a short interruption in November 1272).

[12] In this capacity, he participated in private initiative Hungarian incursions into Austria and Moravia, Ottokar's realms in February 1273, along with Matthew Csák, Denis Péc, Joachim and Amadeus Gutkeled.

Ivan, who, in order to gain time, offered a duel with a third of himself against three knights including Henry, Count of Pfannberg, successfully defended Kapuvár (German: Valbach) by destroying the dam at the river Vág (Váh) and flooding the surrounding area.

[18] In this capacity, Kőszegi was responsible for the territory between the river Drava and the Kapela Mountains, while Gutkeled administered the Adriatic Coast (the dignity of Ban of Croatia emerged from that time in the Kingdom of Hungary).

Rudolf informed the Hungarian king on Ivan's attempt and requested Ladislaus to persuade the rebellious baron to compensate for the damage caused in previous years for his realms and himself offered assistance in redressing common grievances.

[8] According to the bishop's letter to the other prelates in the realm, Ivan Kőszegi held the ispánates of Orbász (Vrbas), Szana, Garics (Podgaric) and Gecske (Gacka) in Slavonia and Croatia during the excommunication, and also usurped the Gerzence (Garešnica) lordship from the diocese.

[30] Since the early 1280s, Nicholas and Ivan Kőszegi extended their influence over Sopron County, taking advantage of that both powerful leaders of the neighboring Csák clan, Matthew II and Peter had died by 1283 or 1284.

Taking advantage of their inaction, Ivan Kőszegi and his troops (including Cumans or Pechenegs) encircled the enemy and shot their camp with arrows, while they did not undertake an open confrontation with the Austrians' heavy cavalry.

[37][35] Simultaneously, the Kőszegis' allies, the Borsa brothers – Roland and James – arrived from Transtisia; they joint troops defeated Ladislaus' army at the river Zsitva (Žitava) in March.

The archbishop complained that his subject, abbot Henry entered the duke's service as captain-general of Styria and threatened those clergymen with excommunication, who held secular positions in the ducal court.

[37] Duke Albert launched a massive royal campaign ("Güssing Feud"; German: Güssinger Fehde) with his 15,000-size army against the Kőszegis and their familiares' castles and forts in the spring of 1289.

The Austrians captured at least 30 fortresses and settlements along the western borders, including Nagymarton (Mattersburg), Németújvár (Güssing), Sopron (Ödenburg), Kabold (Kobersdorf), Szalónak (Stadtschlaining), Rohonc (Rechnitz) and Óvár (Altenburg).

[20][51] As Rudolf I of Germany attempted to bestow Hungary on his own son, Duke Albert of Austria, on 31 August 1290, Andrew III also had temporary interest community with the Kőszegis.

After the failure of the diplomatic negotiations between the two realms, Ivan and his two brothers participated in the king's military campaign in the summer of 1291,[44] when the Hungarian troops invaded Austria, forcing Albert to withdraw his garrisons from the towns and fortresses – including Pressburg and Sopron – that he had captured two years before, many of which were held by the Kőszegis before their conquest.

[53] After a few months of tension, the Kőszegi brothers rose up in open rebellion against Andrew in spring 1292, acknowledging the late Ladislaus' nephew, Charles Martel of Anjou, as King of Hungary.

As a political gesture, his father Charles II of Naples perpetually donated Vas and Sopron counties to Ivan and his son Gregory, as fiefdoms, which element of feudalism was unaccustomed in Hungary.

[54] Simultaneously, in a letter, Charles Martel's mother Queen Mary of Naples authorized "her beloved follower" Ivan to launch a war against Andrew, "the usurper of the Hungarian throne and his accomplices".

The royal troops subdued the rebellion by July, but Ivan Kőszegi captured and imprisoned Andrew III during his journey to Slavonia for a brief time in August, as the Annales Mellicenses recorded.

[59] After the close of the 1298 diet, Andrew III entered into a formal alliance with five influential barons – Amadeus Aba, Stephen Ákos, Dominic Rátót, Demetrius Balassa and Paul Szécs – who stated that they were willing to support him against the "rebellious lords", which term definitely primarily covered Matthew Csák and the Kőszegi brothers.

Ivan Kőszegi was a central figure of the subsequent period of interregnum, which lasted for seven years and various claimants – Charles of Anjou, Wenceslaus of Bohemia, and Otto of Bavaria – fought for the Hungarian throne.

Majority of the historians, including Gyula Kristó and Jenő Szűcs, considered, these barons, for instance, Matthew Csák, Amadeus Aba, Ivan Kőszegi and Stephen Ákos were arbitrarily styled themselves palatines, usurping the position, which marked its devaluation.

According to the narration of the Steirische Reimchronik, after Wenceslaus II met the Hungarian delegation in Hodonín in the summer of 1301, the Bohemian king sent his envoy to Kőszegi and invited him to a personal meeting.

On the following day, Kőszegi accepted the king's offer, also referring to the generosity of the late Ottokar II, Wenceslaus' father, who had create him knight of Teutonic Order during his exile decades earlier.

[73] However, Kőszegi's main motivation behind his support was to defend his province against the House of Habsburg; Wenceslaus' opponent Charles of Anjou was the nephew of Duke Albert, who intended to enter alliance with his maternal relatives.

While Buda fell into Wenceslaus' arms, the castle of Esztergom was conquered by Ivan Kőszegi, expelling its pro-Angevin residents, including his former ally, Archbishop Gregory Bicskei.

Historian Tamás Kádár argues Matthew Csák, who also left Wenceslaus in the same period but did not support Charles either, had tensions with Ivan over the affiliation of Komárom County, inheriting their rivalry from their fathers, and their conflicts of interest caused his departure.

[80] According to the short-spoken Anonymi Leobiensis Chronicon, Charles launched a royal campaign against Ivan Kőszegi in 1305; receiving assistance from Rudolf III, he captured the rebellious lord' three unidentified forts.