Qigong came before Tai Chi Chuan and were individual exercises used for health and wellbeing. Each exercise had its own specific health benefits and were often given as individual exercises as a prescription by Chinese Doctors. Sometimes groups of exercises were put together for a specific aim. One of the oldest sets is the 8 Pieces of the Brocade which was put together by an army general to strengthen his men and keep them fit for battle. The same general also created a seated/bed version for soldiers recovering from battle and/or to be done in the winter to prepare the body before they got out of bed into the cold climate.
Around 500 CE, in the Liang Dynasty, Qigong was adopted by various martial artists to increase stamina and power. For the most part, the breathing, concentration and agility were assets to the warriors and improved their wellbeing.
In the 1970's Kung Fu came to the West via movies and everyone wanted to be Bruce Lee! Most good western tai chi schools taught a complete package, Kung Fu, Qigong (energy work/development), Tai Chi Forms, breathing exercises (Kaimen - Chinese yoga and Daoyin- respiration therapy, we now know as Qigong), massage, meditation, Chinese medicine. The word 'Tai Chi' has become recognised in the West as gentle flowing movements. However, in more recent years a further split has been made. Tai Chi is now generally seen in the West as the Tai Chi Form (The Flowing dance) and Qigong known as the standing breathing exercises.