  We investigate the magnetic reorientation in a two-dimensional anisotropic
antiferromagnet due to a transverse magnetic field. Using a many-body Green's
function approach, we show that the magnetization component perpendicular to
the applied field (and along the easy-axis of the antiferromagnet) initially
increases with increasing field strength. We show that this unexpected result
arises from the suppression of quantum and thermal fluctuations in the
antiferromagnet. Above the N\'eel temperature, this effect leads to a
reappearance of a magnetic moment along the easy-axis.
