Peter held these positions until the sudden death of Stephen V in August 1272, after that he was replaced by Reynold Básztély, as Master of the stewards. In 1271, Peter took part in a military campaign against Ottokar II of Bohemia, who violated the two-year truce, which had been concluded in 1270. He fought in the sieges of Pressburg and Moson. Stephen V won a decisive victory near the Rábca River. Peter's bravery and heroism during the battles had been documented by the royal charter of 1274. According to this, he saved the life of his former enemy, Béla, Duke of Macsó, who lost his horse in the battle. During the time when tensions emerged between Béla IV and his son, Stephen, two rival baronial groups developed, one of them was led by Henry I Kőszegi ("Henry the Great"), also involving the Gutkeled and Geregye clans, while the Trencsén branch of the Csák clan dominated the second group. Following the coronation of Stephen V in 1270, leaders of Béla IV's party fled to abroad from the potential retaliations, however they returned to Hungary, when the crown passed to the minor Ladislaus IV in August 1272. During the nominal regency of Queen Elizabeth the Cuman both sides wished to take part in the exercise of power. The rivalry between the two parties characterized the following years. According to historian Bálint Hóman, twelve "changes of government" took place in the first five regnal years of Ladislaus IV. Peter I regained his influence in June 1273, when he received land donations from the king, for instance Szenic in Nyitra County (today: "Senica, Slovakia"). However Peter lost this estate in early 1274, when the Kőszegi baronial group enjoyed the confidence of Ladislaus IV again. The Csák brothers (Matthew II and Peter) and his allies successfully removed Joachim Gutkeled and Henry Kőszegi from power by the summer of 1274. However the two disgraced lords decided to capture and imprison Ladislaus and the Queen Mother in June 1274. Although Peter Csák liberated the king and his mother, Gutkeled and Kőszegi captured Ladislaus's younger brother, Andrew, and took him to Slavonia.