IN 1867, Marx wrote to Johann Becker that Das Kapital would be “without question the most terrible MISSILE that has yet been hurled at the heads of the bourgeoisie.”

Ever since, Capital has armed the working class with the theory of surplus value, explaining why and how labour power is exploited to the profit of the capitalist class.

Since 1945, for example, the average production worker in Britain or the US has spent anywhere between 2.4 and 3.3 hours of an eight-hour working day performing unpaid labour, creating surplus value over and above the value of the commodities that can be purchased by their daily wage.

Together with Value, Price and Profit — the text of Marx’s contribution to the debate within the International Working Men’s Association in 1865 — Capital explains why the wages struggle is both necessary and fruitful.

Volumes I and III show how — alongside workers producing commodities for capitalist profit — commercial and clerical workers are exploited as well, performing unpaid surplus labour even though they might not be creating surplus value.

Here is the basis for working-class unity across every sector against capitalism.

Capital also exposes the various ways in which employers seek to maximise surplus value through longer hours, higher productivity, devalued wages and the super-exploitation of female, migrant and foreign labour.

Echoing Robert Owen and Karl Marx, the First and Second Internationals proclaimed the eight-hour day as the main demand of worldwide demonstrations on May 1. Today, the struggle to win or defend the eight-hour day or 40-hour week continues around the world.

However, while improvements and reforms might be won, they will be sustained only by the economic and political strength of the organised working class. As Marx warned in Capital, capitalists are driven by competition, mechanisation and the tendency of the rate of profit to fall to seek ways of maximising the surplus labour and value extracted from workers.

Marxist political economy has thus educated and inspired generations of trade unionists, socialists and communists to organise and mobilise millions of workers for many thousands of battles over pay, hours and working conditions.

The result has been a general rise in working-class living standards in the developed and developing countries over the past 150 years.

Marx explained much else about capitalism yesterday and today: about crisis, mechanisation, alienation at work and the “disposable industrial reserve army” of labour.

His treatment of “fictitious capital” and the financial markets anticipated not only the 1929 Wall Street Crash, but also the Great Crash of 2007-8.

Capital’s analysis of capitalist centralisation and concentration — accumulation of more and more capital in fewer and bigger monopolies — anticipated the rise of today’s giant capitalist corporations.

In 2017, monopoly corporations made up more than half of the world’s biggest 150 economic entities by gross domestic product or sales revenue. Each of the three biggest (Wal-Mart, Toyota Motor and Volkswagen) had a turnover bigger than the GDP of at least 150 of the world’s countries.

While these companies straddle the globalised economy, almost all of them have a home country whose national state power is usually exercised in their interests.

Thirty-eight of the biggest 100 transnational corporations are based in the US, with 12 in China (many of them in social ownership), 12 in Japan, eight in Germany, seven in France and six in Britain.

On the basis of Marxist political economy, Lenin and others developed our understanding of the new era of finance capital, state monopoly capitalism and imperialism.

This, in turn, deepened and broadened the struggle for national liberation struggles against colonialism, profoundly affecting the course of world development.

Thanks to Marx and Capital, too, many millions of workers have come to understand that they must fight not only the effects of their exploitation, but its cause.

As Value, Price and Profit put it, these “unavoidable guerilla fights” will always have to be waged against the encroachments of capital and the market, for as long as capitalism itself exists.

Therefore, the working class should not merely alleviate the symptoms — it should eradicate the disease, by abolishing capitalism and constructing the new, communist mode of production.

The subsequent overthrow of different forms of bourgeois power in Russia, eastern Europe, China, south-east Asia and Cuba has had the most profound impact on world development.

Working-class state power, economic planning, social ownership and massive investment in the means of production — as advocated in Capital — enabled the Soviet Union and eastern Europe to achieve higher levels of economic growth, equality and social security than in comparative capitalist countries, all without the super-exploitation of colonies or semi-colonies; the Soviet Union saved Europe from capitalist-fascist barbarism; and the socialist countries rendered invaluable support to national liberation movements around the world.

Today, on a similar economic and political basis, China is emerging as an economic superpower and Cuba’s social and cultural achievements exceed those of almost all developing and many developed capitalist countries.

Working people around the world can now learn from a wide range of experiences of winning state power and building socialism, as the lower stage of communism.

These include immensely positive experiences as well as the failures to resolve problems of resource allocation, innovation, productivity and incentives.

Towards the end of Volume I of Capital, Marx laid bare capitalism’s fundamental contradiction, that between the vast social character of production and the private monopoly ownership of the means of production.

Capitalism’s class system — dominated by the narrow interests of a small class of powerful shareholders — holds back further development of society’s productive forces, which could be deployed to resolve most if not all of humanity’s basic social, economic and environmental problems.

Hence Marx’s belief that the capitalist mode of production and its state power would be overthrown by the growing revolt of the working class, which he said was “a class always increasing in numbers, and disciplined, united, organised by the very mechanism of the process of capitalist production itself.”

The working class has indeed grown since Capital was first published — from 300 million to around 3.4 billion today. We have 268 million workers in trade unions affiliated to the World Federation of Trade Unions or the International Confederation of Trade Unions, with another 322 million in the All-China Federation.

That makes more than half a billion organised workers, more and more of them women whose additional exploitation was reported in detail in Volume I of Capital.

We have the international communist movement and other parties and movements of the left.

Since 1867, the objective conditions have matured for abolishing a capitalist mode of production now characterised by global exploitation, grotesque inequality, parasitic “financialisation,” economic crisis, massive unemployment and underemployment, imperialist militarisation and war, forced mass migrations, food insecurity, undernutrition, environmental degradation, corporate corruption, energy crisis, global warming and destabilised weather systems.

The urgent necessity for Marxists and communists in the 21st century is to ensure that the subjective conditions — working-class organisation, revolutionary leadership and strategy, political consciousness — are sufficient for the historic tasks ahead.

Robert Griffiths is general secretary of the Communist Party of Britain.