Shadow Foreign Secretary and former Deputy Foreign Secretary, Rt Hon Andrew Mitchell MP writes for ConservativeHome.

I’ve been around a while. Long enough to have served alongside politicians from the wartime generation, whose battlefield stories my rookie mind lapped up as a young MP in the 1980s.  Hard truths were learned and imparted by the people who lived through and fought in the Second World War. These were also simple truths. Appeasement must never be an option. Red lines must never be crossed.

Bullies must always be faced down.

The west has been tested several times since 1945. The second half of the 20th century may not have been as bloody as the first, but the Cold War upped the ante in terrifying ways. In 1962 The Cuban Missile Crisis brought the world to the brink of nuclear confrontation, after the US discovered the Soviet Union was installing nuclear tipped missiles on the island of Cuba, eye-wateringly close to Florida.

President John F Kennedy was clear that this was unacceptable and ordered a naval blockade of Soviet arms shipments that were heading America’s way. It was a huge gamble. The standoff that followed could have escalated to the point of no return. But this was Kennedy’s red line. And while the world held its breath, the US held its nerve.  Sure enough, Soviet ships retreated. The bully was faced down.

The world has changed since then, but the simple truths still apply.  Russia’s monstrous invasion of Ukraine is a sobering reminder that when tyrants are given an inch, they will grab whatever they can.

One of the single most important things the Conservative Party did during our time in office was to side unequivocally with Ukraine. Under Boris Johnson and Rishi Sunak, we were often the first mover on vital military aid and pushed our allies to go further. We were at the vanguard of the sanctions response, delivered vital humanitarian support and were constantly looking at new ways of constraining Putin’s war machine.

In opposition our support must remain unwavering. Ukraine’s fight is our fight. Freedom and democracy are at stake. These are not abstract principles but the principles that underpin the open international order, the rock upon which the world’s – and our – security and prosperity rests.

We have seen what happens when resolve is weakened. Parallels are often drawn with the appeasement of Nazi Germany through the Munich agreement but there are echoes closer to home today. The west’s failure to act after President Assad used chemical weapons in Syria in 2011, the allies’ timorous response to Russia’s annexation of Crimea in 2014 and the US’ chaotic withdrawal from Afghanistan in 2021, were watched and exploited by Putin. We display weakness and disunity at our peril.

No more.  Victory for the bully would deliver a hammer blow to the most fundamental international rule – that countries must not seize land from others, that the world cannot be remodelled by force. And make no mistake: Ukraine’s fight is existential. It would be no less naïve to believe that Putin will stop in Ukraine than believing Hitler was staying put in the Sudetenland.

The UK must therefore provide further military aid to Ukraine to defend the frontline and protect critical infrastructure. We must supply the weaponry and authorise the use of it in any way that contributes to Ukraine’s defence against aggression, and which complies with international law and the rules-based system. And let us not lose sight of the fact that this is Europe’s war: the UK has a duty to corral our friends on the continent to stay the course.

We must not allow Putin’s bluster to scare us off. He repeatedly threatens war but the truth is we have already suffered a variety of low-level incursions, with espionage, malign cyber activity and even a suspected arson attack. President Zelensky is right to warn that backing down now would only embolden the enemy.

The Ukrainians have shown remarkable courage and tenacity. David is beating back Goliath in ways we never would have imagined when the war began. Now is the time for the world to step up, not step back. As Thatcher reflected after the Falklands: ‘you cannot allow the difficulties to dominate your thinking. You have to set out an iron will to overcome them’. Ukraine can and will win this war if we and our allies give it the necessary support, at speed, and for as long as it takes.

The American diplomat George Kennan famously warned of Soviet expansionism and observed that the Soviet Union was ‘impervious to the logic of reason, but highly sensitive to the logic of force.’ This simple truth remains as relevant today as it was back then.

Ukraine is our test. Our Munich. Our Cuba. Our Falklands. It is a fight for the rules-based international order. We must stand shoulder to shoulder with Ukraine until victory is assured.