According to Truss, the decision to urgently mobilize signals that Putin now realizes Russia isn't on track for victory.
"I think he has been outsmarted by the Ukrainians," said Truss.
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"We've seen the Ukrainians continue to push back against the Russian offensive. And I think he didn't anticipate the strength of reaction from the free world."
The UK leader added that the West should not heed Moscow's "bogus threats," continuing its support of Ukraine, instead.
Read also: Seven key takeaway's from Putin's "partial mobilization"
"If Putin is allowed to succeed, this wouldn't just send a terrible message in Europe, and of course huge threats to the Ukrainian population themselves, but it also would send a message to other authoritarian regimes around the world that it's somehow acceptable… to invade a sovereign nation," she said.
During her recent speech at the UN General Assembly, Truss vowed to keep UK security assistance to Ukraine at current levels or ramp it up even further, adding that London "won't rest" until Ukraine emerges victorious in the war.
Read also: NATO secretary general says Putin's nuke threat is 'dangerous, reckless rhetoric'
The UK has provided $2.5 billion in military aid to Ukraine so far in 2022.