This week’s London arms market – decorously called a “fair” – is a national disgrace and the protesters currently calling attention to it are right. It is a shameless marketplace for global death and destruction, vitiating any work done by British diplomacy in support of a more peaceful world.

The best apology the government can offer is that it “supports jobs”. The same line of argument applies to membership of the EU – and yet the government doesn’t support that job supporting measure.

That a nation should seek to defend itself from external aggression is understandable. Where the country is a British ally, it is reasonable to help it with weapons. This applies to very few countries round the world, and even fewer of those likely to buy the guns, tanks, ships, missiles and drones on display at the Excel centre this week. As anyone who has visited this show in the past knows, it is the most awesome glamorisation of death on the planet.

The claim that the British government sells only for internal security ranks high in Whitehall’s mendacity register. Few of the 35,000 delegates from 68 countries at the “fair” are in the democracy promotion business: witness the attempted sale (now stopped) of prison shackles and electric-shock weapons. Big, high-priced sellers are bombers, drones and vanity warships. Britain should not be weaponising the suppression of dissent in Egypt, Bangladesh, Colombia, Uzbekistan or Saudi Arabia and other Gulf states. For them, national defence might be better termed, regime defence.

The government boasted of record sales of £14bn last year, with 80% going to the Middle East. The tragedy being inflicted on Yemen by Saudi Arabia, major recipient of British weaponry, led to the court of appeal ruling in June that arms sales to the Saudis are illegal. The reality is that Britain and the US are in an arms race with the Russians in this theatre – with no remotely peaceful objective.

Military interventionism remains an obsession of the western powers: Donald Trump, to his credit, has shown some reticence in intervening – and yet he likes to threaten military action around the world and US forces continue to drop bombs on foreign countries. Actions by western powers are based on a belief in their moral superiority and also in their right to impose that superiority on others through force rather than example. In the days of empire, such imposition at least had the quality of effectiveness. Today it has not. The devastation of Afghanistan, Iraq and Libya are only the most prominent examples of intervention failure.

Britain no longer has the capacity to impose peace or democracy on the world. What it does have is the opportunity to set an example in its own democratic behaviour – not exactly a shining one at present. Its foreign policy should not be to aid war – to add fuel to the flames of violence round the world is inexcusable. The arms fair starting in London tomorrow should never be repeated. It would help if the opposition parties said so.

• Simon Jenkins is a Guardian columnist