  High-mass Stars are cosmic engines known to dominate the energetics in the
Milky Way and other galaxies. However, their formation is still not well
understood. Massive, cold, dense clouds, often appearing as Infrared Dark
Clouds (IRDCs), are the nurseries of massive stars. No measurements of magnetic
fields in IRDCs in a state prior to the onset of high-mass star formation
(HMSF) have previously been available, and prevailing HMSF theories do not
consider strong magnetic fields. Here, we report observations of magnetic
fields in two of the most massive IRDCs in the Milky Way. We show that IRDCs
G11.11-0.12 and G0.253+0.016 are strongly magnetized and that the strong
magnetic field is as important as turbulence and gravity for HMSF. The main
dense filament in G11.11-0.12 is perpendicular to the magnetic field, while the
lower density filament merging onto the main filament is parallel to the
magnetic field. The implied magnetic field is strong enough to suppress
fragmentation sufficiently to allow HMSF. Other mechanisms reducing
fragmentation, such as the entrapment of heating from young stars via high mass
surface densities, are not required to facilitate HMSF.
