The following is my summary of previous theories.

The idea to generalize General Relativity to all forces has been around since Einstein. General Relativity explains gravitational force with curved space-time. There is no force acting upon a particle. Particles are following an inertial path or the shortest path possible, a geodesic, on curved space-time. Explaining all other forces with the same concept is very aesthetically pleasing.

Mass causes space-time curvature. So a mass creates a change in the path a particle takes because it warped the space the particle is traveling within. This theory is as accurate in the large scale as QED is in the subatomic scale. It predicts the apparent displacement of a star because of a gravitational field. It also explains why inertial mass is equal to gravitational. A particle following a geodesic path is following an inertial path. There is no gravitational force working as a force at a distance. Only local space curvature determines a particles local trajectory. General Relativity uses a four by four matrix, a tensor, as the basis for calculating space-time curvature.

Einstein attempted to fold EM force into a tensor so that it would be an extension of general relativity. This would unify two separate distinct forces into one theory, Unified Field Theory. This unification became significantly more difficult as additional nuclear forces were added as fundamental forces. There are no intrinsic quantum properties in a tensor. General relativity looks more like a way of correctly calculating an answer rather than the basis for explaining a force. The success of QED using particle interchange as the basis of a force showed much more progress in explaining the properties of an electron and EM fields. The success of QED kept others from pursuing UFT.

Michio Kaku's book 'Hyperspace' explains most of what I know of Kaluza-Klein theories. To have a UFT requires a larger size tensor that corresponds to more physical dimensions. Researchers have added EM and nuclear forces to create a tensor that corresponds to these forces. Their work and ideas have led to present day superstring theories.

Superstring theories start with the concept of additional dimensions from Kaluza-Klein. It takes the additional dimensions and curls them up tightly so they are smaller than any particle. These strings are short. But since they are strings they have natural quantum states built into them. Just like a guitar string has a fundamental and harmonics that can be of only certain values, a superstring also has certain allowable, or quantized, frequencies. The resulting quantum nature of a vibrating string has brought it to the forefront of research. It's roots still lie in UFT.