We present the results of radiation-magnetohydrodynamic simulations of the formation and expansion of H II regions and their surrounding photodissociation regions ( PDRs ) in turbulent , magnetised , molecular clouds on scales of up to 4 parsecs . We include the effects of ionising and non-ionising ultraviolet radiation and x rays from population synthesis models of young star clusters . For all our simulations we find that the H II region expansion reduces the disordered component of the magnetic field , imposing a large-scale order on the field around its border , with the field in the neutral gas tending to lie along the ionisation front , while the field in the ionised gas tends to be perpendicular to the front . The highest pressure compressed neutral and molecular gas is driven towards approximate equipartition between thermal , magnetic , and turbulent energy densities , whereas lower pressure neutral/molecular gas bifurcates into , on the one hand , quiescent , magnetically dominated regions , and , on the other hand , turbulent , demagnetised regions . The ionised gas shows approximate equipartition between thermal and turbulent energy densities , but with magnetic energy densities that are 1 to 3 orders of magnitude lower . A high velocity dispersion ( \sim 8 km s ^ { -1 } ) is maintained in the ionised gas throughout our simulations , despite the mean expansion velocity being significantly lower . The magnetic field does not significantly brake the large-scale H II region expansion on the length and timescales accessible to our simulations , but it does tend to suppress the smallest-scale fragmentation and radiation-driven implosion of neutral/molecular gas that forms globules and pillars at the edge of the H II region . However , the relative luminosity of ionising and non-ionising radiation has a much larger influence than the presence or absence of the magnetic field . When the star cluster radiation field is relatively soft ( as in the case of a lower mass cluster , containing an earliest spectral type of B0.5 ) , then fragmentation is less vigorous and a thick , relatively smooth PDR forms . Accompanying movies are available at http : //youtube.com/user/divBequals0