On the night of 22 February 2014, the most powerful men in Russia gathered in the Kremlin and resolved to seize Crimea from Ukraine.

They would later make elaborate efforts to give their decision a veneer of legitimacy – including by staging a bogus referendum – but that meeting between President Vladimir Putin and his security chiefs was designed to seal the fate of Crimea’s people.

We know this because Mr Putin said as much. In a documentary for Russian television, broadcast in 2015, the President described the sequence of events.

He decided to grab Crimea during that conclave in the Kremlin 3 weeks before the sham referendum. All those claims about how he acted to protect the region’s people or uphold their wishes were, by Mr Putin’s own account, utterly mendacious.

So it was that Russia seized 10,000 square miles from Ukraine and broke the first principle of international law – that countries may not acquire territory or change borders by force.

Mr Putin formally annexed Crimea into the Russian Federation on 18 March 2014. Four years after that event, we should remind ourselves of the enormity of what happened and redouble our determination to stand up for our values and uphold international law.

Russia’s land grab in Crimea amounted to the first forcible annexation of the territory of a European country – and the first forcible redrawing of a European frontier – since 1945.

In the process, Russia broke so many international agreements that listing them all is a challenge. To select a few examples, Mr Putin trampled upon Article 2 of the United Nations Charter, the Helsinki Final Act and the Russia-Ukraine Treaty of Friendship.

He also broke Russia’s specific pledge, contained in the Budapest Memorandum of 1994, to respect the “existing borders of Ukraine” and “refrain from the threat or use of force against the territorial integrity or political independence of Ukraine”.

And after annexing Crimea Mr Putin went still further, igniting and vigorously fanning the flames of conflict in eastern Ukraine. To this day, Russia continues to deploy troops and tanks in a conflagration that has claimed over 10,000 lives and driven 2.3 million people from their homes.

Flight MH17 became another victim of this tragedy when a Russian missile launched from an area controlled by Russian proxy fighters blew this passing airliner out of the sky, killing 298 innocent people, including 10 Britons.

All the while, reports have emerged from Crimea of the oppression of the indigenous Tatar population and the harassment of those opposed to Russian annexation. Despite repeated calls from the UN General Assembly, Russia has refused to allow international human rights monitors to enter the peninsula.

In the end, the security of every nation depends on the essential principle that countries should not change borders or acquire territory by force. That is why the fate of Crimea matters to all of us.

We all have an obligation to stand up to Russia in a measured and resolute way. That means sustaining our Crimea-related sanctions against Russia for as long as the region remains under Kremlin control, and keeping further sanctions in place whilst the Minsk Agreements in eastern Ukraine go unheeded.

These measures are intended to demonstrate that no country, however large, can dismember its neighbour and break international law without consequence.

Nevertheless, while holding fast to our principles, we should engage firmly and purposefully with Russia. We need to communicate with clarity and directness our concern over the Kremlin’s actions.

There is no contradiction between dialogue and deterrence – indeed the one can reinforce the other – as I made clear when I visited Moscow in December. As permanent members of the Security Council, Britain and Russia also share special responsibility for international peace and security.

Our motto with Russia must be to ‘engage but beware’ and both halves of the formula should be pursued with equal resolve. But we must never forget the terrible consequences of that late night gathering in the Kremlin.