[2] He was notable for his outspoken opposition to World War I, which caused his arrest under the Defence of the Realm Act and loss of his teaching post, after which he became a full-time Marxist lecturer and organiser.

Maclean believed that Scottish workers were especially fitted to lead the revolution, and talked of "Celtic communism", inspired by clan spirit.

[4][5][6] His parents spoke Gaelic[7] and he was raised in a Calvinist household, Maclean trained as a schoolteacher under the auspices of the Free Church and then attended part-time classes at the University of Glasgow, graduating with a Master of Arts degree in 1904.

In 1906, Maclean gave a series of speeches in Pollokshaws which led to the formation of an SDF branch there, and through these, he met James D. MacDougall, who became his strongest supporter for the remainder of his life.

[9] As a revolutionary enemy of what he saw as an imperialist war, Maclean was fiercely opposed to the stance adopted by the leadership of the BSP around H. M. Hyndman.

By the time of World War I, his socialism was of a revolutionary nature, although he worked with others on the Clyde Workers' Committee who were more reformist in outlook, such as his friend James Maxton.

During World War I, he was active in anti-war circles and was imprisoned in 1916 for breaching the Defense of the Realm Act,[7] but was released in 1917 after demonstrations following the February Revolution in Russia.

For the full period of my active life I have been a teacher of economics to the working classes, and my contention has always been that capitalism is rotten to its foundations, and must give place to a new society.

Every week the socialist papers kept up the barrage and reminded their readers that in Germany Karl Liebknecht was already free, while in 'democratic' Britain John Maclean was lying in a prison cell being forcibly fed twice a day by an India rubber tube forced down his gullet or up his nose.

'to be stained with a crime in some respects even more horrible and revolting, more callous and cruel, than that which the Governors of Ireland perpetrated on the shattered body of James Connolly?'

Following the Easter Rising he had contacts with members of the Scottish Divisional Board of the Irish Republican Brotherhood[23] In the summer of 1907 he went on a speaking tour of Ireland, here he made friends with Jim Larkin.

[24] He believed that workers in Scotland could develop in a revolutionary direction more swiftly than their comrades in England and Wales, and in 1920 he attempted to found a Scottish Communist Party.

Maclean's call for a Communist Republic of Scotland was based on the belief that traditional Scottish Gaelic society was structured along the lines of "community".

[26] [check quotation syntax] His stay in Peterhead Prison in 1918 caused a considerable deterioration in his health, being force fed through hunger strikes.

[27] Milton quotes a letter that Agnes, his wife, wrote to Edwin C. Fairchild (a leading member of the British Socialist Party): Well, John has been on hunger strike since July.

He left a legacy that has subsequently been claimed by both the Scottish Nationalist and Labour movements, making him rare in this respect amongst Scotland's historical figures.

[32] The magazine Socialist Appeal has labeled him a "Marxist who played an outstanding role in promoting the ideas and cause of Marxism...[and] worked like a Trojan to promote the principles of Marxism amongst the working class of Scotland"[33] The National describes him as "a man who most knowledgeable Scots would consider a legend, indeed an almost mythical Celtic giant of socialism".

[38][39] In 1948, MacDiarmid and Smith (among others) gave readings at a "huge mass meeting" at St. Andrew's Hall in Glasgow, organised by the Scottish-USSR Society to mark the 25th Anniversary of his death.

[40] The Scottish Esperanto poet and novelist John Islay Francis (1924–2012)[41] in his novel La Granda Kaldrono ("The Big Cauldron") published in 1978,[42] describes different attitudes toward the first and the second world wars.

He is referenced in several of the tracks on the album Red Clydeside by folk musicians Alistair Hulett and Dave Swarbrick, and also in the song "Rent Strike" by Thee Faction.

[45] The bosses and the judges united as one man, For Johnny was a menace to their '14 — '18 plan, They wanted men for slaughter in the fields of Armentières, John called upon the people to smash the profiteers

They brought him to the courtroom in Edinburgh town, But still he did not cower, he firmly held his ground, And stoutly he defended, his every word and deed, Five years it was his sentence in the jail at Peterhead Maclean's life is celebrated in the play The Wrong Side of the Law by Ayrshire writer Norman Deeley, dealing with the political and personal struggles that Maclean faced in his fight to establish socialism in Scotland.

The Soviet Union (USSR) honoured Maclean with an avenue in central Leningrad[46] – Maklin Prospekt, which ran north from the Fontanka towards the Moika.

In 1979, on the centenary of his birth, the USSR issued a 4 kopeck commemorative postage stamp depicting Maclean in a portrait by Peter Emilevich Bendel.