Garvan Walshe is a former National and International Security Policy Adviser to the Conservative Party.

“They preferred the safety of the state purchased by their death to personal safety at the price of submission. Thus with their bodies they bore the brunt of battle and perished at the height of glory.”

The men Pericles so extolled died for nothing: Athens lost the Peloponnesian war to Sparta. Its government was overthrown, its empire dismantled and its democratic institutions replaced by the “thirty tyrants.” Fine oratory had stirred the Assembly to fight on, whatever the costs.

When Greek democracy was re-established, it included a new procedure designed to give demagogues pause. With the “graphe paranomon” (illegal proposal), a populist leader could be tried and punished once the consequences of his irresponsible politics became apparent.

On Sunday, Greeks voted for honour and independence. Never mind that the question put could be understood only by experts in the operation of the EU’s financial architecture, or that Alexis Tsipras had lied to them about the effects of their vote. It perhaps stretches things a little to describe their defiance of the institutions formerly known as the Troika as the height of glory, but there is much in it that is noble, heroic and admirable in their assertion of sovereignty and the primacy of democracy.

Greece has spoken with exceptional clarity. It wants to be ‘In Europe, paid for by Europe’.

But, the 27 other EU member states are also democracies. Nine are poorer than Greece because they had the kinds of economic policies that Syriza favours imposed upon them by the NKVD. They don’t see why they should now pay for Greek communism. Greece will be hit by the biggest cause of disillusion in politics: the limits of democratic sovereignty. No single country can get its way on its own – none is strong enough, not even the United States. Leaders have to bargain, co-operate, work through institutions, and build up reputations for trust, effectiveness and reliability.

Though Angela Merkel has understood this, David Cameron does not. This may be clearest in military spending. At last year’s NATO summit in Wales, he again urged NATO members to spend 2 per cent of their GDP on defence, to raise their game and put money into European security, but he hasn’t increased defence spending himself. The Government may protest that they have met the 2 per cent defence budget target, and if one takes a generous view of what counts as defence expenditure this might be true. But it is not the 2 per cent target but the change in the level of British military commitment that matters. If the amount spent on defence was adequate two years ago, before Russia invaded Ukraine and the IS was proclaimed, it clearly isn’t any more.

Indeed, the blind spot is broader than defence; it extends across all foreign policy. He didn’t send the Foreign Secretary to the negotiations after Ukraine’s revolution, and Britain now plays a back seat in confronting Russian imperialism there. Michael Fallon was right to suggest that British planes should be allowed to attack ISIS on both sides of the Iraqi-Syrian border. But though a step in the right direction, this would not amount to a serious strategy to contain the “generational threat” that the Primie Minister considers Islamism to be, because his Prime Minister is not willing use the political will required to get one adopted. The ancient Greeks had a word for holding a belief sincerely but failing to act in accordance with it – akrasia – and they considered a serious vice.

Nowhere is Cameron’s akrasia more evident than in his response to last week’s terrorist attack in Tunisia. I don’t for a moment doubt his will to confront Islamist extremism, or protect British citizens from violence and disorder. He owes his political position to very few concrete beliefs, but he takes these duties of his office extremely seriously. What he lacks (but Angela Merkel, for instance, possesses) is the focus to make the political system produce the means to meet what he is convinced are the obligations a British Prime Minister owes his people.

If anything is an international matter, Islamist terrorism is, yet the government’s response to this latest attack has consisted chiefly of domestic security measures. These may or may not be necessary, but they won’t stop an angry Tunisian man with a gun killing tourists in his country. As long as Britons travel abroad, conduct international business and take part in the cultural and economic life of the world, protecting them requires Britain to get stuck in, and be known for getting stuck in, to the hard work and messy compromises with other countries in international society.

This can’t be done by domestic security: Britain is only an island in the literal geographic sense. If in Victorian times she maintained her security by empire, she must now do it by sustained engagement with the rest of the world.