On December 21, 1963, serious violence erupted in Nicosia when a Greek Cypriot police patrol, ostensibly checking identification documents, stopped a Turkish Cypriot couple on the edge of the Turkish quarter. A hostile crowd gathered, shots were fired, and two Turkish Cypriots were killed. As the news spread, members of the underground organizations began firing and taking hostages. North of Nicosia, Turkish forces occupied a strong position at St. Hilarion Castle, dominating the road to Kyrenia on the northern coast. The road became a principal combat area as both sides fought to control it. Much intercommunal fighting occurred in Nicosia along the line separating the Greek and Turkish quarters of the city (known later as the Green Line). Turkish Cypriots were not concentrated in one area, but lived throughout the island, making their position precarious. Vice-President Küçük and Turkish Cypriot ministers and members of the House of Representatives ceased participating in the government.

In January 1964, after an inconclusive conference in London among representatives of Britain, Greece, Turkey, and the two Cypriot communities, UN Secretary General U Thant, at the request of the Cyprus government, sent a special representative to the island. After receiving a firsthand report in February, the Security Council authorized a peace-keeping force under the direction of the secretary general. Advance units reached Cyprus in March, and by May the United Nations Peace-keeping Force in Cyprus (UNFICYP) totaled about 6,500 troops. Originally authorized for a three-month period, the force, at decreased strength, was still in position in the early 1990s.

Severe intercommunal fighting occurred in March and April 1964. When the worst of the fighting was over, Turkish Cypriots--sometimes of their own volition and at other times forced by the TMT--began moving from isolated rural areas and mixed villages into enclaves. Before long, a substantial portion of the island's Turkish Cypriot population was crowded into the Turkish quarter of Nicosia in tents and hastily constructed shacks. Slum conditions resulted from the serious overcrowding. All necessities as well as utilities had to be brought in through the Greek Cypriot lines. Many Turkish Cypriots who had not moved into Nicosia gave up their land and houses for the security of other enclaves.

In June 1964, the House of Representatives, functioning with only its Greek Cypriot members, passed a bill establishing the National Guard, in which all Cypriot males between the ages of eighteen and fifty-nine were liable to compulsory service. The right of Cypriots to bear arms was then limited to this National Guard and to the police. Invited by Makarios, General Grivas returned to Cyprus in June to assume command of the National Guard; the purpose of the new law was to curb the proliferation of Greek Cypriot irregular bands and bring them under control in an organization commanded by the prestigious Grivas. Turks and Turkish Cypriots meanwhile charged that large numbers of Greek regular troops were being clandestinely infiltrated into the island to lend professionalism to the National Guard. Turkey began military preparations for an invasion of the island. A brutally frank warning from United States president Lyndon B. Johnson to Prime Minister Ismet Inönü caused the Turks to call off the invasion. In August, however, Turkish jets attacked Greek Cypriot forces besieging Turkish Cypriot villages on the northwestern coast near Kokkina.

In July, veteran United States diplomat Dean Acheson met with Greek and Turkish representatives in Geneva. From this meeting emerged what became known as the Acheson Plan, according to which Greek Cypriots would have enosis and Greece was to award the Aegean island of Kastelorrizon to Turkey and compensate Turkish Cypriots wishing to emigrate. Secure Turkish enclaves and a Turkish sovereign military base area were to be provided on Cyprus. Makarios rejected the plan, because it called for what he saw as a modified form of partition.

Throughout 1964 and later, President Makarios and the Greek Cypriot leadership adopted the view that the establishment of UNFICYP by the UN Security Council had set aside the rights of intervention granted to the guarantor powers--Britain, Greece, and Turkey--by the Treaty of Guarantee. The Turkish leadership, on the other hand, contended that the Security Council action had reinforced the provisions of the treaty. These diametrically opposed views illustrated the basic Greek Cypriot and Turkish Cypriot positions; the former holding that the constitution and the other provisions of the treaties were flexible and subject to change under changing conditions, and the latter, that they were fixed agreements, not subject to change.