Thanos Thanos:
Given the nature of the times, and the fact that the FLQ had begun murdering people, Trudeau had every reason to believe that a civil insurrection was breaking out in Quebec, and that if the federal government didn't act immediately events would rapidly get worse. The way he smashed it down before it spiraled out of control was one of the few things, if not the only thing, I ever admired him for.
Maybe, maybe not. The FLQ were certainly a problem that needed to be stopped, but preemptively deploying the military to stop a potential civil insurrection in Canada's largest city at the time set horrible precedent for Canada's future.
The Americans at least were facing significant issues (major riots and a civil war) that brought forth martial law. Canada's civil authorities were not compromised or overwhelmed during the FLQ Crisis to require the deployment of the military in Montreal, the arrests of hundreds without legitimate cause, and the suppression of the freedoms and liberties of all citizens of Montreal, anglophones or francophones.
$1:
It was really no different, and just as necessary, as the British government outlawing the British Union of Fascists or the US clamping down on the German-American Bund at the outbreak of WW2. They were all-but-declared enemies of the government, just as the FLQ was, and they had to be dealt with.
The British Union of Fascists was only made illegal in 1940, during the Second War II, thus making it more acceptable for such a situation to occur, and the German-American Bund was prosecuted and suppressed by HUAC, without the intervention of the American military.
Canada wasn't at war at the time, like in the case of Britain, nor did Trudeau allow civil authorities (like HUAC) to carry out their responsibilities.
$1:
Democracies can withstand an occasional bruising now and then. What is fatal to them is when anti-social and anti-government activities aren't immediately confronted and the perpetrators stopped. None of our rights, codified or otherwise, or respect for the rights of others obligate us to remain in a state of paralysis when violence and other dangerous threats to the social order make themselves apparent.
Except, in my view, as much as the FLQ was a threat, it was not a threat that could paralyze the whole city of Montreal, or Quebec, let alone Canada, due to possible violence. They needed to be dealt with, but we already have the police to deal with such situations. The FLQ Crisis never reached a level where the liberties of over a million Canadians had to be curtailed and suspended to restore order.