Not necessarily. For example, the absence of any AQ attacks on US soil since 9/11 does not necessarily mean that such attacks have been deterred. They may no longer be necessary. Suppose (and I don't think it an unlikely supposition) that AQ's intention was to use the 9/11 attack to pull the US into an action in Afghanistan that could lead to a war of attrition. This would target the greatest military weakness of the US - lack of long-term political will - and create one of the few scenarios in which a military defeat for the US is a real possibility.
If this is the case, additional attacks on the US would be completely counterproductive: having initiated the war of attrition, AQ's task now is to wear down America's will to fight, and further direct attacks on the US would bolster and sustain that will.
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