Key dates in tensions between Protestants and Catholics:
1921: Northern Ireland is formed from six provinces that opt out of a self-government agreement offered by Britain and accepted by the southern part of Ireland.
1967-68: Northern Ireland's Catholics push for voting rights and demonstrate against unionist power in Northern Ireland.
1972: Direct rule of Northern Ireland from London begins. Violence against British troops quickly follows. Police report more than 1,400 bombings; 470 people are killed during the conflict's bloodiest year.
1994: Mostly Catholic Irish Republican Army announces a cease-fire. It lasts until February 1996, when the IRA sets off a bomb in London, killing two.
1998: USA helps broker the Good Friday peace accord in Northern Ireland. Voters in Ireland and Northern Ireland endorse it.
2005: IRA agreement to disarm clears way for implementation of the agreement.
2006: Great Britain and Ireland prod Northern Ireland's politicians to move forward on home rule and shared power.
2007: Voters elect a new assembly on March 7, giving the Protestant and Catholic political parties the power to form an executive government.
Monday: If Northern Ireland doesn't form a government by this deadline set by Britain and Ireland, Britain will continue to rule it in consultation with Ireland.
Reporting by Melanie Eversley, USATODAY.
Sources: Wire reports and USA TODAY research | 