  We study the transition between anti-parallel and component collisionless
magnetic reconnection with 2D particle-in-cell simulations. The primary finding
is that a guide field \approx 0.1 times as strong as the asymptotic
reconnecting field -- roughly the field strength at which the electron Larmor
radius is comparable to the width of the electron current layer -- is
sufficient to magnetize the electrons in the vicinity of the x-line, thus
causing significant changes to the structure of the electron dissipation
region. This implies that great care should be exercised before concluding that
magnetospheric reconnection is antiparallel. We also find that even for such
weak guide fields strong inward-flowing electron beams form in the vicinity of
the magnetic separatrices and Buneman-unstable distribution functions arise at
the x-line itself. As in the calculations of {\it Hesse et al.} [2002] and {\it
Yin and Winske} [2003], the non-gyrotropic elements of the electron pressure
tensor play the dominant role in decoupling the electrons from the magnetic
field at the x-line, regardless of the magnitude of the guide field and the
associated strong variations in the pressure tensor's spatial structure.
Despite these changes, and consistent with previous work, the reconnection rate
does not vary appreciably with the strength of the guide field as it changes
between 0 and a value equal to the asymptotic reversed field.
