  We use idealised N-body simulations of equilibrium discs in live and static
haloes to study how dark matter co-evolution impacts the assembly of stellar
particles into a bar and the halo response. Initial conditions correspond to a
marginally unstable disc according to commonly used disc stability criteria,
and are evolved for the equivalent of about 150 disc dynamical times (10Gyr).
An extensive convergence study ensures accurate modelling of the bar formation
process. Live haloes lead to the formation of a strong bar, but the same disc
remains unbarred when evolved in a static halo. Neither seeded disc
instabilities, nor longer (60Gyr) simulations result in the formation of a bar
when the halo is static. When the live halo is replaced with a static analogue
at later times the previously robust bar slowly dissipates, suggesting: (1) the
co-evolution of the disc and halo is critical for the assembly and long-term
survival of bars in marginally unstable discs; and (2) global disc stability
criteria must be modified for discs in the presence of live haloes. In our live
halo runs, a "dark bar" grows synchronously with the stellar bar. Processes
that inhibit the transfer of angular momentum between the halo and disc may
stabilise a galaxy against bar formation, and can lead to the dissolution of
the bar itself. This raises further questions about the puzzling stability of
observed discs that are marginally unstable, but unbarred.
