The EU leader said that the bloc should have paid more attention to warnings about Putin.
"One lesson from this war is we should have listened to those who know Putin," said Ursula von der Leyen, president of the European Commission.
She said that those warnings came from many of the bloc's own members.
Many of the countries she identified were once part of the Soviet Union.
She paid tribute to the murdered Russian journalist Anna Politkovskaya and other dissident journalists.
Von der Leyen said that they had been told for a long time that Putin would not stop.
—Visegrád 24 (@visegrad24) September 15, 2022
Since WWII, the largest military deployment in Europe has been by Russia.
In the face of Ukraine's pro- European leanings, Putin repeatedly signaled that he considered Ukrainians and Russians to be one people. He was warned of his "imperial ambitions" back in 2021.
Seven months have passed since Putin sent his tanks across the border and began a war.
The EU's dependence on Russian energy was thrust into the spotlight by the war.
In her speech, von der Leyen said that the countries which had warned about Putin acted accordingly.
Around 40% of the EU's natural gas consumption came from Russia in 2021.
She said that it was down to 9%. Climate change makes energy demand worse.
The issue has been forced by Russia's decision to flow gas to Europe through theNord Stream 1 line, an issue that Russia initially attributed to repair problems, but which Putin now says is dependent on the removal of war sanctions.
Other EU officials have said that the bloc should have acted sooner on Ukraine.
The EU's high representative for foreign affairs and security policy said earlier this month that the EU should have started training Ukrainian troops a year ago.
We regret that we did not. We regret that we didn't fulfill this request in August.