He had concluded that peaceful means would never bring Britain to respect American sovereignty and independence

From the excellent Perilous Fight by Stephen Budiansky, an account of the naval aspects of the War of 1812. Page 90.

What friends and enemies alike failed to grasp was that the kind of man who quietly worked out solutions to complex problems in the privacy of his study and his mind could be a man of stubborn resolve once he determined what those solutions were. And the fact was that as early as the spring of 1811 he [Madison] had concluded that peaceful means would never bring Britain to respect American sovereignty and independence; no option remained but war. His challenge now was to slowly, cautiously, and deliberately build the political case for what would inevitably be seen as a total about-face in a policy that for a decade had been most closely associated with no one but Madison himself.