The coup was instrumental in bringing to power a new developmentalist elite and in laying the foundations for the rapid industrialization of South Korea under Park's leadership, but its legacy is controversial for the suppression of democracy and civil liberties it entailed, and the purges enacted in its wake.

Although the singularly problematic economic and political climate of the Second Republic encouraged a military intervention, the roots of the coup go back to the late Rhee period.

These "liberation aristocrats" formed the bulk of the political class, encompassing both Rhee's supporters and his rivals in the Democratic Party, which advanced a vision of society broadly similar to his own.

[7] The lack of development under Rhee provoked a growing nationalistic intellectual reaction which called for a radical restructuring of society and a thorough political and economic reorganization.

Park Chung Hee, the later leader of the May Coup who at that time was a second-tier army officer with decidedly ambiguous political leanings,[8][9] was heavily influenced by this unfolding intellectual reaction.

[11] The Second Republic adopted a parliamentary system, with a figurehead president as head of state; executive power was effectively vested in the prime minister and cabinet.

[12] Purges of Rhee's appointees were rendered ineffective in the public eye by Chang's manipulation of the suspect list to favour wealthy businessmen and powerful generals.

[13] Although Rhee had been removed and a democratic constitution instituted, the "liberation aristocrats" remained in power, and the worsening problems facing South Korea were proving insurmountable for the new government.

[18] Park had attended all three institutions, and was uniquely positioned to lead what would become the coup coalition, with his extensive ties among both the senior commanders of the army and the younger factions.

[18] After the overthrow of the Rhee regime and the institution of the Second Republic, the reformists, led by KMA alumni, began to call for the senior commanders to be held to account for complicity in the rigging of the 1960 and 1956 presidential elections.

[19] Park, relatively high-ranking as Major General, threw himself into the spotlight by declaring his support for the reformists and demanding the resignation of Army Chief of Staff Song Yo-chan on May 2.

[8][20] On September 24, 16 colonels, led by Kim Jong-pil, demanded the resignation of Chairman of the Combined Chiefs of Staff Choi Yong-hui in an incident known as the "revolt against seniors" (하극상 사건; 下剋上事件; hageuksang sageon).

From May to October 1960, however, Park assembled a variety of officers to organize a new plan for a coup, largely on the basis of his ties with other graduates of the Manchurian Military Academy.

Furthermore, it was decided that Park would focus on building support for the coup among other generals, while the other core members would recruit younger officers and construct revolutionary cells within and outside Seoul.

[26] In March 1961, the core group met at the Chungmu-jang Restaurant in Seoul, and fixed the date April 19 for the coup, expecting significant disturbances on that day due to its being the anniversary of the revolution that had overthrown Rhee's regime.

Pervasive unrealized rumours of the imminence of a military coup also contributed to Chang Myon's decision, and the report on the May 12 plan was dismissed as a false alarm.

[30] With the Sixth District Army now secure under his control, Park chose Colonel Kim Jae-chun to organize the vanguard of the occupation of Seoul and dispatched a message to Chang Do-yong, instructing him to definitively join the coup or suffer the consequences of association with the civilian government.

By 4:15am, after a brief exchange of fire with loyalist military police who were guarding the bridge across the Han, Park's forces had occupied the administrative buildings of all three branches of government.

Commander Lee Han-lim of the First Army had prepared to mobilize the reserves to suppress the coup, but backed down to prevent an opportunity for a North Korean attack.

[38] The SCNR was formalized as a junta of the 30 highest-ranking military officers initially arranged in 14 subcommittees, and assumed a wide-ranging responsibility that included the powers to promulgate laws, appoint cabinet posts, and oversee the functioning of the administration as a whole.

On June 6, the SCNR promulgated the Law Regarding Extraordinary Measures for National Reconstruction, which stripped Chang of his posts of Defense Minister and Army Chief of Staff.

Finally, on July 3, Chang himself was arrested on a charge of conspiracy to carry out a counter-coup, and the June 10 law was amended to allow Park to assume the office of chairman both of the SCNR and its Standing Committee.

[36] On June 24, American Ambassador Samuel D. Berger arrived in Seoul, and reportedly informed Park that the United States was interested in publicly supporting his government, but required the cessation of "purges and recriminations".

[42] The purges of state ministries were escalated by the announcement on July 20 of a policy programme aiming at the forced retirement of almost 41,000 "excess" bureaucrats and the reduction of the number of civil servants by 200,000.

[43] The purges of the government apparatus, Park's triumph in the power struggles that followed the May coup, and his eventual election as civilian president in 1963 set the stage for the consolidation of his developmental regime.