The situation following Ceylon’s long-delayed and somewhat botched independence were… surprisingly stable. Sure, there was a brief (and mostly peaceful) communist insurrection, a British military occupation, and ethnic strife, but harmony was eventually achieved! The Sinhala and Tamil populations were integrated and given their rightful language privileges under the benevolent eye of the moderate UNP government, while Ceylon plotted a peaceful nonaligned course through world politics, backed by a rather mediocre mixed economy.

Unfortunately, the Ceylon success story was not to last. The policy of the ruling UNP pacified the Tamil population, but they failed to consider that the Sinhala supermajority of the island might develop a highly aggravated victim complex. The masses of (mostly illiterate) Sinhalas, especially the youth, were swiftly convinced that far from being the dominant force in the country, they were actually the most oppressed group, even more than the impoverished Indian Tamil estate workers.

In the March 1972 elections, the UNP was predictably punished for their position. Huge numbers of lower-class Sinhala voters and many members of the UNP’s upper class conservative faction defected to Bandaranaike’s SMS, which promised the institution of the Sinhala Only act and the “protection of the Buddhist religion and Sinhalese culture.” However, when the results came in, the SMS and their allies were a shocking two seats away from forming a majority. The remaining opposition, formed of the moderate faction of the UNP, the Tamils, and the Communists, were unwilling to consider forming a coalition to support Sinhala only.

Meanwhile, the SMS’s near victory had triggered a nationwide hartal, or general strike, led by left wing parties opposing a return to Sinhala chauvinism. At this point Bandaranaike faced a decision: pursue Sinhala Only, go to another election, and potentially trigger ethnic riots, or compromise and form a government right away.

Bandaranaike, never exactly fond of violence, chose to back down. Instead, he proposed a solution which would fulfill most of his goals while maintaining popular support: replacing the constitution. Bandaranaike began discussions with the communist parties and the radical Tamil Federal Party to replace the British written Soudbury constitution with a new republican document establishing a Socialist Republic of Sri Lanka, with Sinhala as the primary official language and Tamil as the secondary official language with extensive legal protections. The proposed constitution would also establish a limited degree of federalized government through Regional Councils.

Under the banner of socialism, Bandaranaike also proposed nationalizing the estates and redistributing the land to the mostly Tamil laborers, instituting workers democracy for industrial firms, and beginning a large job-creation program for the mostly Sinhala unemployed youth through land reclamation and expensive state investment in infrastructure and industry.

This arrangement proved satisfactory to the people Bandaranaike hoped it would - left wingers were thrilled to enshrine leftist economic policy into the constitution and civil service, and most Tamils were at least satisfied to have their rights protected by the constitution rather than a basic law. The working class, especially Tamil estate workers, were also satisfied to have a leader standing up for their interests after the perceived elitist rule of the UNP.

However, as word of the secret negotiations began to leak, it drew strong opposition from both wings of the UNP, and especially the conservative Sinhala nationalist faction led by J.R. Jayewardene. Jayewardene organized massive marches of Sinhala nationalists, previously a strong SMS constituency, against the proposed constitution. The moderate Tamil Congress also opposed the bill, citing concerns about the balance of Tamil and Sinhala seats in Parliament.

The protracted negotiations and civil unrest created a pervasive atmosphere of instability in Colombo. It seemed as though Parliament was pursuing narrow partisan concerns over real governance and sacrificing the unity of the nation for political points, while the streets burned every night. Still, Bandaranaike pushed ahead, determined to establish his policies as the foundation of the nation. As the negotiations went on, Bandaranaike made plans to form a government in the meantime with as many leftist parties as possible to enact land reform legislation and use the resulting public support to secure a larger majority.

However, he was preempted by the military. The officer corps overwhelmingly consisted of conservative, Christian, English speaking officers heavily influenced by Britain. Furthermore, the army had mostly inherited the traditions and organization of the all-volunteer Ceylon Defense Force despite British efforts to train a regular military, and thus the majority of military manpower came from part time reservists with ties to the old plantation class, who had recently been mobilized to contain civil unrest.

On the night of November 11th, 1972, a committee of officers led by Major General B. R. Heyn, Volunteer Force Commandant F. C. de Saram, Navy Commander D. V. Hunter, Army Colonel J. H. V. de Alwis, and senior police officers Osmund De Silvia, C. C. Dissanayake and Sydney de Zoysa, led several battalions of regulars, volunteers, and police in seizing the majority of government buildings in Colombo and arresting Communist and Socialist leaders. No armed opposition was encountered, and by morning the coup was free to announce that “the Armed Forces had stepped in to protect the national constitution and end the period of political infighting and deadlock.” Furthermore, it was promised that “democracy would be restored as soon as possible when the populist agitators causing unrest were stopped,” and in a concession to Sinhalese nationalism, “the armed forces are concerned with protecting the majority rights of the Sinhala people which the current government sought to violate.”

The initial coup went off seemingly without a hitch, but immediately things started going wrong. It was discovered that senior SMS figure F. R. D. Bandaranaike had been out of town and had escaped arrest. Furthermore, the strength of the communist movement was clearly not as damaged as expected, since counter protests and strikes immediately began throughout much of the nation.

However, a lot also went right. The coup was able to secure all communications centers and transport before word could spread quickly. With the time that action bought, the coup plotters were able to gain much-needed legitimacy by establishing an interim cabinet with prominent UNP statesmen like John Kotelawala and J. R. Jayewardene, which in turn secured the cooperation of much of the civil service. Furthermore, the coup has been somewhat successful in co-opting Sinhala nationalists by pledging to prioritize the Sinhala language over Tamil, and even exploring the possibility of instituting Sinhala Only as a policy.

Armed violence has not sprung up yet, likely the result of the demilitarized state of the country more than anything else, but mass strikes by Tamil estate workers and urban laborers have crippled the economy, and the coup has so far been unsuccessful in finding scabs. However, the military remains in effective control of the country’s territory and the state apparatus.

TLDR:

SMS wins a plurality of seats and opts to form a coalition government with the communists and create a new left wing constitution instead of going full Sinhala nationalist.

The elite-populated military launches a coup against the government in the name of capitalism, with a window dressing of hardline Sinhala nationalism.

The coup succeeds and takes control of the government, but faces strong unarmed opposition from left wing political constituencies.