Chiropractic care is the biggest field of complementary medicine, with 1/3 of people with lower back pain treated by a chiropractor, and is licensed in all 50 states. Unlike many other areas of medicine, the history of chiropractic is well-documented. We can even narrow it down to a specific date.
On September 18th, 1895, Daniel David (D.D.) Palmer noticed that a vertebra in the back of a partially deaf janitor was out of its position. The janitor, Harvey Lillard, told him that he had felt it pop when it moved and he had been deaf ever since. D.D. Palmer performed the first spinal adjustment by pushing the vertebra back into its position, reportedly curing Lillard's deafness. From there, Palmer founded the field of chiropractic (which means "handwork").
While this story makes it seem as though chiropractic care appeared out of nowhere, the technique that Palmer performed and the philosophy behind chiropractic care did stem from other practices. Palmer was knowledgeable of bonesetting, which was a folk technique that centered around the idea of subluxations and of moving bones back into their correct positions.
He also believed in magnetic healing, which was the idea that in order to be healthy, energy needed to move through the body unimpeded. Palmer credited the human nervous system and the spinal cord with being the basis of the body's energy. Palmer merged these two fields together to create the overarching philosophy of chiropractic care.
Since its inception, chiropractic care has been in conflict with mainstream medicine. It took many decades and legal trials for chiropractic care to be accepted and licensed. But, nowadays, it is often seen almost a mainstream medicine itself. Chiropractic treatments are often covered by insurance and there have been thousands of scientific studies into its effectiveness.
Within chiropractic care, there has also been a fair amount of conflict and disagreement. Most chiropractors have moved away from some of the ideas first proposed by Palmer. The idea of magnetic healing has been dropped by many and the focus on vertebrae being out of alignment as the sole cause of problems related to the back and spine has been broadened by many to include the mobility of other joints and how the curvature of the spine and posture might affect the muscular system and cause pain.
Some chiropractors, known as "straights", perform spinal manipulation and chiropractic techniques exclusively. Whereas other chiropractors, known as "mixers", use a much wider range of therapeutic techniques alongside chiropractic to help their patients. These can include physical therapy, massage, nutritional advice, ultrasound, acupuncture, and many more. These latter chiropractors take a holistic approach to patient care, using all of the tools available to help improve their quality of life, but there is some debate over whether they should still be called "chiropractors".
The history of chiropractic care is tumultuous and varied. It is hard to imagine, now that chiropractic care is so established, that it was viewed as dangerous nonsense not so long ago. It is a very new branch of complementary medicine when compared to the much longer history of conventional medicine, so it is likely that many of the teething problems about what treatments should be offered, what the mechanism is behind how chiropractic care works, etc, will be ironed out and the field will continue to grow.