For the first time since the TNA-NJPW partnership in 2008, on March 27, 2014, it was reported that Styles would return to New Japan Pro-Wrestling and had signed a contract with the company. Styles' return came on April 6 at Invasion Attack, where he attacked IWGP Heavyweight Champion Kazuchika Okada after his match and laid him out with the "Styles Clash" and challenged Okada to a title match, before revealing himself as the newest member of Bullet Club, establishing himself as a villain in the process. Following Invasion Attack, with Styles still working a full schedule on the American independent circuit and only booked to work NJPW's larger events, Karl Anderson was positioned as the new leader of Bullet Club. However, Styles was considered the leader of the Ring of Honor (ROH) version of Bullet Club, a role which was also given to him by NJPW by the end of 2015. Styles wrestled his first match under a New Japan contract on May 3 at Wrestling Dontaku, where he defeated Okada with help from Yujiro Takahashi to become the new IWGP Heavyweight Champion. With this victory, Styles became the sixth "gaijin" (non-Japanese) champion in the title's history and the first American champion since Brock Lesnar in 2005. The win also made Styles the shortest IWGP Heavyweight Champion in history. On May 17, Styles made his first successful title defense in a three-way match with Okada and Michael Elgin at the NJPW/ROH co-produced War of the Worlds event in New York City. On May 25 at Back to the Yokohama Arena, Styles defeated Okada for his second successful title defense. Styles suffered his first loss in NJPW on July 21, when he was defeated by Okada in his first match in the 2014 G1 Climax tournament. Out of his nine remaining matches in the tournament, Styles lost only one, but failed to advance to the finals due to his loss against Okada in the head-to-head match. On October 13 at King of Pro-Wrestling, Styles lost the IWGP Heavyweight Championship to Hiroshi Tanahashi in his third defense, after the returning Yoshitatsu stopped Bullet Club's newest member, Jeff Jarrett, from interfering in the match.