  By viewing the electron as a wavepacket in the positive energy spectrum of
the Dirac equation, we are able to achieve a much clearer understanding of its
behavior under weak electromagnetic fields. The intrinsic spin magnetic moment
is found to be established from the self-rotation of the wavepacket. A
non-canonical structure is also exhibited in the equations of motion due to
non- Abelian geometric phases of the Dirac spinors. The wavepacket energy can
be expressed simply in terms of the kinetic, electrostatic, and Zeeman terms
only. This can be transformed into an effective quantum Hamiltonian by a novel
scheme, and reproduces the Pauli Hamiltonian with all-order relativistic
corrections.
