When the Russian opposition leader Boris Nemtsov was assassinated in sight of the Kremlin last Friday night, it shocked even those of us who thought we had lost the ability to be shocked by events in Vladimir Putin’s Russia. When Russian forces moved into Ukraine and Putin annexed Crimea a year ago, it was also a terrible shock to a world that had grown too comfortable with the belief that the days of changing Europe’s borders by force were long over. But we must cease to be surprised by the violence and hatred emanating from Russia today if we are to combat it successfully.

When the shock subsides and the evidence is examined, it is clear no one should truly have been surprised by either horrific event. Boris, with whom I worked closely for many years, often talked of the violent ends faced by those who spoke out against Putin. We all knew what could happen to any of us at any time, and a few months after I last left Russia, in February 2013, I decided I would not return.

Police states are very good at keeping a monopoly on violence, and Putin’s Russia is no exception. When the victim is a former Russian deputy prime minister and a prominent critic of the regime, and his murder takes place in a wide open area right next to the Kremlin, the chance that it occurred without the involvement of Russia’s security services is vanishingly small. Boris was always under personal and electronic surveillance, but we are supposed to believe that his escort had the night off, and all nearby CCTV cameras happened to be down for repairs that day.

“But this is Mr Kasparov’s personal interpretation!” shouted one alarmed BBC presenter when I shared those observations in a live interview this week. “But the Russian government has categorically denied any involvement!” cried another. I accept that the things I say are my personal interpretation, but why is the BBC positioning itself as Putin’s defence attorney?

The man has a record; my insinuations are hardly far-fetched. Why cite the official statements of a dictatorship that lies and spreads propaganda at every turn without challenging them? It’s a good example of how the conventions of an open society are exploited by less scrupulous regimes. It represents the culture of engagement and appeasement that has come to replace the harder line of the cold war. But the time for unreciprocated fair play is over.

Yesterday I was in Washington DC, speaking to a US Senate subcommittee about how and why the Russian dictator must be stopped. Nearly every head in the room nodded in agreement as I and other invitees – such as the former Georgian president Mikheil Saakashvili – discussed the global danger presented by Putin’s increasingly belligerent regime.

The EU and US did little to take a stand that might have altered Putin’s course when it would have been relatively easy

The subcommittee circulated a detailed timeline of Russia’s slide back into dictatorship during Putin’s 15 years in power, noting each murder of a critic or journalist, each media outlet closure, each rigged election. I noted that it could easily have produced a parallel timeline of how the leaders of the western world treated the perpetrator of these atrocities at the time. As Putin’s human rights violations – and border violations – accumulated, the European Union and America did very little to take a strong stand that might have altered Putin’s course when it would have been relatively easy to do.

Consider the lesson Putin learned the last time Russian tanks crossed a border. In 2008 Russia provoked a military conflict over Abkhazia and South Ossetia, the quasi-independent areas it had helped carve out of Georgia when the USSR fell. Russian forces nearly reached Tbilisi before they turned back, and those supposedly independent regions are little more than Russia-controlled enclaves and a thorn in the side of the Georgians.

If you don’t remember what punishment was meted out to Russia for invading its tiny neighbour, it’s because there wasn’t any. Not only was Russia not punished over Georgia, a few months later Putin was rewarded by newly sworn-in President Barack Obama and his secretary of state, Hillary Clinton. Their infamous “Russian reset” wiped the slate clean, despite no real change in Kremlin behaviour. Based on that experience, it is no wonder Putin expresses surprise at the relatively robust response to his assault on Ukraine.

An appropriately strong reaction to Putin’s aggression in Ukraine and his brutality in Russia will require a battle on many fronts. Putin, like other modern autocrats, has an advantage that the Soviet leadership could never have dreamed of: deep economic and political engagement with the free world. The naive idea was that the free world would use economic and social ties to gradually liberalise authoritarian states. In practice, the authoritarian states have abused this access and economic interdependency to spread their corruption while cracking down ever harder at home.

For the most immediate crisis, Ukraine must be comprehensively supported militarily and economically. Defensive weapons to raise the political costs for Putin in Russia are essential. To address two of the most popular straw-man arguments: it isn’t necessary to defeat the entire Russian army, or start the third world war. Putin cannot afford to look like a loser, which is why he maintains the feeble illusion that Russian forces aren’t fighting in Ukraine.

Putin’s oligarch supporters must be forced to choose between giving him up and painful quarantine

Inflicting enough damage to pierce that illusion is enough. Putin is already blaming the US and Nato for everything in Ukraine so there can be no escalation in that regard. Ukraine may seem far away, but it is the frontline of a war the United States, the United Kingdom and the rest of the free world are fighting, whether they admit it or not.

Engagement has failed, and it is time for sterner stuff, the successful cold war recipe of isolation and condemnation. Putin’s oligarch supporters must be forced to choose between giving him up and painful quarantine. The Russian oligarchs are supporting a sponsor of terror in Ukraine, and there is no shortage of existing laws to prosecute such activity.

The opposition movement that Boris and I believed in, and that Boris died for, should be openly supported, the way the west once championed the Soviet dissidents. Ronald Reagan told those of us behind the iron curtain that he knew it was our leaders, not us, who were his adversaries. We listened and it mattered, and it should matter again. More than 100,000 people rallied to mourn Boris in Moscow last Sunday, a number that gives the lie to Putin’s meaningless approval numbers. Tell these people, and the millions too afraid to march, that they have a choice.

Russia will always be my country, but it is difficult to imagine returning while Putin is still in the Kremlin. I will continue to do whatever I can to draw support to the cause of returning Russia to the path of democracy. The western administrations that have passively watched Putin turn Russia back into a dictatorship – and invade his neighbours – are out of excuses. The next death in Ukraine or the next murder of a dissident in Moscow will be blood on their hands – and no investigation will be necessary.