Here's a look at what you need to know about Yemen, which is located on the southwestern tip of the Arabian Peninsula and shares a border with Saudi Arabia and Oman.
Facts: Yemen is one of the poorest countries in the Arab world, due to declining oil resources.
Yemen's economy depends on foreign aid and money sent home by Yemenis who are employed in other parts of the Arabian Peninsula.
Most Yemenis are farmers, herders or craftsmen.
May 22, 1990 - The Republic of Yemen is created from unification of North Yemen, the People's Democratic Republic of Yemen (PDRY), and South Yemen, or the Yemen Arab Republic (YAR).
About Yemen: (CIA World Fact Book) Land Area: 527,968 sq km (slightly larger than twice the size of Wyoming) Population: 25,408,288 (July 2013 est.) Median age: 18.5 years Capital: Sanaa Ethnic Groups: Predominantly Arab, also Afro-Arab, South Asian, European Religion: Muslim including Shaf'i (Sunni) and Zaydi (Shia), small numbers of Jewish, Christian, Hindu GDP: $54.85 billion (2012 est.) GDP per capita: $2,200 (2012 est.) Unemployment: 35% (2003 est.)
Timeline: May 22, 1990 - The Republic of Yemen is created from unification of North Yemen, the People's Democratic Republic of Yemen (PDRY), and South Yemen, or the Yemen Arab Republic (YAR).
April 1993 - Elections are held after postponement due to political turmoil.
May 1994 - A civil war between northerners and southerners begins due to disagreements between supporters of Yemen's president, Ali Abdullah Saleh, from North Yemen, and its vice president, Ali Salim al-Baid, from South Yemen. An estimated 5,000 people are killed. Troops loyal to Saleh win the war in July.
1998 - Worsening economic conditions contribute to increased militant Islamist activity in the late 1990s. In 1998, 12 tourists were kidnapped, four of whom were murdered. Thereafter, troubles with al-Qaeda and similar groups increased.
September 1999 - Saleh wins Yemen's first direct election. He is re-elected in 2006.
2000 - Saudi Arabia and Yemen agree to a delineation of their border.
October 12, 2000 - The U.S.S. Cole, at anchor in Aden, is bombed by al Qaeda, killing 17 U.S. sailors.
October 2002 - Al Qaeda is blamed for an attack on the Limburg, a French-flagged oil tanker off the Yemeni coast.
August 2004 - A Zaydi rebellion erupts in northern Yemen.
February 2006 - Twenty three inmates escape from a prison in Sanaa. Thirteen are convicted al Qaeda members, including Jamal al-Badawi, who plotted the attack on the U.S.S. Cole.
September 17, 2008 - A suicide bombing at the U.S. embassy in Sanaa kills 16, including six terrorists and one American. It is the fourth time since 2003 that the embassy has been attacked.
December 28, 2009 - A Yemeni-based arm of al Qaeda, al Qaeda in the Arabian Peninsula (AQAP), claims responsibility for a failed bombing on a Northwest Airlines flight from Amsterdam to Detroit, Michigan on Christmas.
January 2, 2010 - U.S. President Barack Obama announces a new counter-terrorism partnership with Yemen, involving intelligence sharing, military training, and joint attacks.
January 3-5, 2010 - The U.S. and the UK temporarily close their embassies in Sanaa due to terror concerns.
February 11, 2011 - Protests begin in Yemen, inspired by the revolution in Egypt that ousted Hosni Mubarak. The demonstrations continue for months, while crackdowns on protesters lead to civilian deaths.
March 20, 2011 - President Ali Abdullah Saleh dismisses his cabinet.
March 23, 2011 - President Saleh offers to step down as president by the end of year. Opposition groups reject his proposal.
April 23, 2011 - President Saleh tentatively accepts a deal, arranged by the Gulf Cooperation Council, to leave office. The agreement states if Saleh resigns the presidency within 30 days, he and members of his regime will be given total immunity.
May 22, 2011 - Saleh again refuses to sign the deal to step down. The Gulf Cooperation Council suspends its efforts to mediate a deal between Saleh and opposition forces.