  Earlier work presented a spacetime path formalism for relativistic quantum
mechanics arising naturally from the fundamental principles of the Born
probability rule, superposition, and spacetime translation invariance. The
resulting formalism can be seen as a foundation for a number of previous
parameterized approaches to relativistic quantum mechanics in the literature.
Because time is treated similarly to the three space coordinates, rather than
as an evolution parameter, such approaches have proved particularly useful in
the study of quantum gravity and cosmology. The present paper extends the
foundational spacetime path formalism to include massive, nonscalar particles
of any (integer or half-integer) spin. This is done by generalizing the
principle of translational invariance used in the scalar case to the principle
of full Poincare invariance, leading to a formulation for the nonscalar
propagator in terms of a path integral over the Poincare group. Once the
difficulty of the non-compactness of the component Lorentz group is dealt with,
the subsequent development is remarkably parallel to the scalar case. This
allows the formalism to retain a clear probabilistic interpretation throughout,
with a natural reduction to non-relativistic quantum mechanics closely related
to the well known generalized Foldy-Wouthuysen transformation.
