1910: Of Nigeria's many ethnic groups, the West's Yoruba, the Southeast's Igbo, and the North's Hausa represent the largest regional divisions. The three are divided by language, ethnicity, religious beliefs, and cultural traditions. British colonial administration encourages their deep cultural separation, which reinforces a natural impediment to national and political unity.

1911-1921: Governor-General Frederick Lugard implements a local format of indirect rule, borrowed from its perceived successful implementation in India and Sudan. The British colonial administration uses select local leaders to carry out colonial regulations and laws, thereby minimizing direct contact with the people and also opposition to the policies and intrusion of a foreign authority.

1922-1939: A nationalist movement led by such charismatic figures as Herbert Macaulay and Nnamdi Azikiwe gives voice to anticolonial dissent. Local legislative councils evolve under the hegemony of colonial rule and exchange ideas about nationalization and administrative participation with other councils. Though their demands are rebuffed, a lasting nationalist consciousness is born.

1940-1945: The Nigerian National Council is formed in 1944 in response to the colonial administration's refusal to consider nationalist demands. With Herbert Macaulay as president and Nnamdi Azikiwe as secretary general, the council opens membership to all Nigerians in an effort at unity. With a goal of self-governance, the council shuns its past passive willingness to work within the current administration.

1946-1950: The British begin to yield to Nigeria's nationalist movement and mounting postwar pressure to decolonize. Constitutional revisions in 1947 allow for the creation of a central legislative body. The following year, large-scale reforms are implemented. Steps are taken to "Nigerianize" the civil service, democratize the local legislatures, and expand social services.

1951-1958: The London Conference of 1953 yields a constitution for an independent Nigeria. It calls for the creation of a federation with a strong centralized government and regional administrations led by Nigerian-born premiers and ministers. But regional conflict dominates the political environment, and progress is thwarted by continual scrambling for position in anticipation of independence.

1959-1962: Even as Nigeria proclaims independence in 1960, regional conflicts worsen. Population-based regional representation at the federal level makes census-data collection and potential restructuring of geographical regions contentious. The populous North helps elect Prime Minister Balewa. His tenuous coalition government is unable to pursue unified national interests in a bitterly divided climate.

1963-1966: Nigeria's governmental structure is modeled on the British parliamentary system and includes three distinct branches -- legislative, judiciary, and executive -- which exist at both federal and regional levels. Newly elected President Nnamdi Azikiwe fails to end increasingly violent regional clashes that result in deadly rioting and the eventual overthrow of the civilian government.

1967-1970: Eastern leaders declare the Independent Republic of Biafra after thousands of Igbo settlers die in ethnic clashes in the Muslim-dominated North. The leaders demand greater autonomy and the authority to retain tax and oil revenues. Lt. Col. Gowon's offer to divide the country into 12 states to prevent political domination by the North is rejected, and a brutally divisive civil war begins.

1971-1974: Following the civil war, Gowon's military regime continues to centralize power in the federal government. In 1974 he announces that his efforts toward stabilizing the political system will delay a return to civilian rule. Gowon promises to draft a new constitution subject to approval of the people, but a widespread feeling of disillusionment with his regime envelops the country.

1975-1979: Murtala Muhammad and Olusegun Obasanjo's military regimes stress commitment to civilian rule. Each continues a four-year transition program to restructure federal government, draft a new constitution, create new states, and hold state and federal elections by 1979. Policy initiatives include decentralizing power, forming national political parties, and combating inflation by reducing money supply.

1980-1984: President Alhaji Shehu Shagari governs from minority status amid struggle with opposition leaders. His administration continues the corrupt practices of post-civil war governments, subordinating long-term social and economic development programs to projects that provide potential for personal profit. Despite the turmoil, Shagari wins a second term in controversial 1983 elections.

1985-1993: Ibrahim Babangida assumes power in 1985 with an empty promise to return Nigeria to civil rule. His decision to annul a 1993 election won by Moshood Abiola throws the country into a political crisis and forces his resignation. Violent clashes between Muslims and Christians increase dramatically when Nigeria registers as a member of the Organization of the Islamic Conference in 1986.

1994-1998: Gen. Sani Abacha reinforces military rule. In spite of claims that he is preparing for a return to civilian government, he replaces elected legislatures with military appointees. Abacha lifts the ban on political activity that he himself imposed, but he imprisons Abiola and obstructs the formation of legitimate political parties. Abacha's abrupt death is followed by Abiola's death in prison.

1999: Abdulsalami Abubakar guides the country into presidential elections; Olusegun Obasanjo, who led the 1976 military government, wins. Oil production and exportation become lightning rods for unrest as demonstrators use violent protests and strikes to underscore widespread socioeconomic inequalities, including a lack of access to basic resources, rampant unemployment, and environmental degradation.

2000-2003: The Northern states implement sharia, underscoring religious and regional differences and challenging the constitution, notably with death sentences for women convicted of adultery. Violent community protests over oil production and economic inequalities continue in the Niger Delta. President Obasanjo trounces his opponents in his bid for reelection, but charges of vote rigging abound.

2007:  Umaru Yar'Adua was chosen as the presidential candidate of the ruling PDP.  Yar'Adua's success in the primary was attributed to the support of incumbent President Olusegun Obasanjo; At the time of his nomination he was an obscure figure on the national stage, and has been described as a "puppet" of Obasanjo who could not have won the nomination under fair circumstances. Shortly after winning the nomination, Yar'Adua chose Goodluck Jonathan, governor of Bayelsa State, as his vice-presidential candidate. Yar'Adua was one of few serving governors with a spotless record, devoid of any suspicions or charges of corruption. He also belongs to the People's Democratic Movement (PDM) - a powerful political block founded by his late brother, Shehu Musa Yar'Adua, who was also Obasanjo's vice president during his military rule.