There is some debate over who "founded" Hapkido but it is clear that many of the principles come from Yong-Sul Choi. (1904-1986)
Choi was a native Korean that was taken to Japan at an early age and eventually was taken into Grandmaster Sokaku Takeda's (the head of Dato Ryu Aki-Jutsu) household. It is not clear what relationship Choi held in Takeda's household. Most llikely Choi was a servant to the household yet it has been stated that Choi was one of Takeda's Chief Instructors. Unfortunately no documentation exists to substantiate it, which is not surprising since during this time Japanese considered Koreans to be inferior and discrimination was common. At the end of World War II, before Tekada's death, he ordered that Choi be returned to Korea. After returning to Korea Granmaster Yong-Sul Choi began teaching Martial Arts.
One of Grandmaster Choi's students, Grandmaster Han-Jae Ji is considered by some to be the founder of Hapkido. He took what he learned from grandmaster Choi and combined in with techniques learned from Korean Taoists to form Hapkido, the name which passed to his master, Yong-Sul Choi.Grandmaster Ji trained many of the prominent Hapkido masters including Bong Soo Han, who was the first to bring Hapkido to the United States. He also shared techniques with Bruce Lee and appeared as the adversary of Bruce Lee in the movie "The Game Of Death".
Bong-soo Han brought much notariety to Hapkido when he choreographed and performed fight scenes in the movie "Billy Jack". Bong-Soo Han recentlly passed away after a lifetime devotion to Martial Arts.
Modren US Hapkido is a variation founded by Grandmaster Chan K Yu of St. Joseph, Missouri. Grandmaster Yu based his techniques on ancient Korean mudo. He came to the United States and began instructing in 1974. He serves as a technical advisor of Hapkido, Judo and Tae Kwon Do. He coached the 1978 U.S.A Tae Kwon Do team in PanAmerican games and serves as an international referee.
These are many interpretations of the philosophy of Hapkido but most are similar in nature. There are three basic principles. The first is Harmony, or non-resistance. Meet your opponent in such a way as to redirect the force, using minimum force to counter theirs. The second is circular motion. This emphasizes fluid motion, one event leading to the next and avoids linear and direct motion. The third is the water theory. This is the theory in which one prepetually attacks his opponents defenses to achieve total penatration. Think of water as it flows over rocks, continually cutting a channel until it is a vast canyon. Hapkido seeks to redirect the opponents force and aim it so it is utilized to our advantage.
The philosophy of Hapkido is based around the East Asian concept of um and yang. These are two great forces that coexist in a balance yet always changing, one unable to exist without the other. This is also referred to as yin and yang. For example, to have light you must have dark, to have low you must have high, etc.
A practitioner of Hapkido seeks to maintain a balance. We seek to meet force with a moderation of flexibility and strength to achieve desired results. Thus we have "the way of harmonious power."
Hapkido is a well rounded Martial Art, blending punches, kicks, throws, and loint locks. It is designed to use the opponent's force against them, giving opportunity to smaller practitioners. It is designed as a defensive art, directed at common simple attacks.
In today's world we have seen a phenominal growth in the world of mixed martial arts. It is my opinion that Hapkido was one of the first to accept and utilize techniques of many different Martial Arts (Tae Kwon Fo, Akido, Judo etc.) anf apply them to meaningful and practical self defense methods. No art is undefeatable. The secret lies in keeping an open mind and continually growing as we practice Martial Arts.