The word “realpolitik” comes from the German for “politics of reality”, and is usually associated with the conduct of international relations – generally less tied to principles and legitimating discourses than national or local politics are. Violations of international law on the part of powerful states occur so often that the international arena is an excellent place for observing how politics really work.

The term “reason of state”, for its part, is usually identified with exceptional measures taken by the state to protect itself, regardless of whether they contradict the official goals of the state, its ethics, or its own laws; those who invoke reason of state for their political decisions are therefore called statists.

Among the precursors or developers of these ideas are usually counted the Chinese military strategist Sun Tzu, Machiavelli, Cardinal Richelieu, Bismarck and Carl Schmitt. What such disparate figures have in common is their ability to understand that politics is basically the art of power, of how to obtain and retain it. This apparently obvious fact may seem more doubtful when we describe state and local political scenes, but it is much more transparent in the international arena.

The difference between a patriot and a terrorist often comes down to the difference between winning and losing

In Gillo Pontecorvo’s film The Battle of Algiers, Colonel Mathieu, commander of the French troops in Algeria fighting the pro-independence National Liberation Front (FLN), is asked by some journalists whether his men torture their suspects to get information. Mathieu replies that the duty of soldiers is to win, and interrogation techniques simply respond to this imperative.

He suggests that rather than wonder whether torture is legitimate, the journalists ought to ask themselves whether Algeria should be French. If they agree that it should (and even the left thinks it should, he adds), then they should let the army get on with its job, which is to win. A little later, Pontecorvo inverts the scene by showing the captured FLN leader, Larbi Ben M’hidi, being asked about the basket bombs the guerrillas use against the civilian population. Ben M’hidi responds ironically that they would like nothing better than to attack France instead, and proposes to the French a weapons swap.

These scenes confront us with the Weberian ethic of responsibility that impels all political enterprises – that is, the justification of one’s own project whatever its ideological underpinnings. The Zionists will say they’re defending the free world; the Islamists that Allah is on their side; the USSR claimed to be acting on behalf of socialism and the world proletariat.

There are as many ideological pretexts as there are projects. What matters is not the end that would justify the means, so much as who decides which wars are just, when a military intervention is humanitarian, or when torture and killing are required by reason of state.

As Robert McNamara put it in the documentary The Fog of War, had the US lost the second world war he would have been tried as a war criminal for organising the bombing of Japanese civilians. In other words, the difference between a patriot and a terrorist often comes down to the difference between winning and losing. Ask the British Queen why she found herself shaking hands with Martin McGuinness, IRA veteran and member of the Sinn Féin party that currently shares power in Northern Ireland.

Once we understand that realpolitik and reason of state explain political actions better than any kind of legitimising ideology, we will recognise the cynicism of all those who present politics as a fairytale about goodies and baddies.

Is this to say that morality has no place in politics? By no means. But in order to change something, first you have to understand how it works.

• Pablo Iglesias is leader of Spain’s Podemos party. This is an edited extract from Politics in a Time of Crisis: Podemos and the Future of a Democratic Future