In an interview with US broadcaster CNN, Sikorski said Putin had underestimated how European democracies would respond to Russia's actions.
He said Europe initially failed to fully recognise that Russia had effectively been waging a war against it for about a decade, but that perception has now changed.
Sikorski, who is also one of Poland's deputy prime ministers, pointed to recent attacks targeting the UK and Germany, saying they demonstrate the Europe-wide scale of the Russian threat.
He added that attacks in Eastern Europe are helping build support for increased defence spending.
Sikorski said Europe has been bearing the costs of supporting Ukraine entirely since January last year, as the United States is no longer contributing financially.
He described the spending as an investment in Europe's security.
"It is costly, but deterring Putin if he were to conquer Ukraine would be even more costly," he said. "So personally, I believe it is a good investment."
Sikorski said Putin is now trying to salvage something from the war but ultimately cannot conquer Ukraine.
He identified two factors that Putin could potentially use to try to change the situation: another mobilisation in Russia and a harsh winter.
On mobilisation, Sikorski suggested Putin might believe that sending another 500,000 Russian troops into the war could finally break the Ukrainian front. Sikorski rejected that possibility.
He also said Russia is attacking civilian power infrastructure, including district heating plants, apparently hoping that harsh winter conditions and disrupted heating would force Ukrainians to abandon their cities and weaken their willingness to continue fighting.
Sikorski argued that such a strategy would not significantly affect Ukraine's ability to maintain its positions along the front.
He told CNN that if both the mobilisation and winter strategies fail, conditions for a ceasefire and serious negotiations could emerge sometime next year.
(sp/gs)
Source: IAR/PAP