  We present results of angle-dependent magnetoresistance measurements and
direct optical images of underdoped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ which reveal
partial detwinning by action of a 14T magnetic field. Driven by a substantial
magneto-elastic coupling, this result provides evidence for an electronic
origin of the lattice distortion in underdoped iron pnictides. The observed
anisotropy in these partially detwinned samples implies a substantial in-plane
electronic anisotropy in the broken symmetry state, with a smaller resistivity
along the antiferromagnetic ordering direction.
