Decided until

For more than twelve months now, we, the Negro citizens of Montgomery have
been engaged in a non-violent protest against injustices and indignities experienced
on city buses. We came to see that, in the long run, it is more honorable to
walk in dignity than ride in humiliation. So in a quiet dignified manner, we decided
to substitute tired feet for tired souls, and walk the streets of Montgomery
until the sagging walls of injustice ( ) crushed by the battering rams of
surging justice.

__________________

In the "( )", what phrases should be most appropriate?

I guess the correct answer should be "were".

I wonder what the opinions of Forum Gurus are.