(CNSNews.com) - Twelve years ago today, nineteen al Qaeda terrorists  hijacked four U.S. commercial airliners and flew them into the World  Trade Towers, the Pentagon and a field in Pennsylvania.

In the war that Congress authorized against al Qaeda only three days  after that attack, the vast majority of the U.S. casualties have  occurred in the last four and a half years during the presidency of  Barack Obama.

In fact, according to the CNSNews.com database of U.S. casualties in  Afghanistan, 73 percent of all U.S. Afghan War casualties have occurred  since Jan. 20, 2009 when Obama was inaugurated.

The 91 U.S. casualties in Afghanistan so far in 2013 are more than those that occurred in  the first two full calendar years of the war (2002 and 2003) combined, when 30  and 31 U.S. troops were killed there.

On Sept. 14, 2001, Congress approved a resolution authorizing the  president “to use all necessary and appropriate force against those  nations, organizations, or persons he determines planned, authorized,  committed, or aided the terrorist attacks that occurred on September 11,  2001, or harbored such organizations or persons.”

By October 2001, U.S. forces were engaged in Afghanistan, seeking to  remove al Qaeda from the sanctuary it had used there to launch the Sept.  11, 2001 attack.

Since then, most of the leaders and participants in the 9/11  terrorist attacks have been killed or captured. But the United States  not only remains at war in Afghanistan, it continues to suffer  significant casualties there.

In total, 2,144 U.S. military personnel have given their lives  fighting  in and around Afghanistan as part of Operation Enduring  Freedom.

1,575 of the U.S. military personnel who have sacrificed their lives  in this cause were killed during the presidency of Barack Obama.

That means 73 percent of the casualties in the Afghan War have happened on Barack Obama’s watch.

Under President George W. Bush, from 2001 until Jan. 20, 2009, 569 U.S. military  personnel were killed in and around Afghanistan fighting in Operation  Enduring Freedom.

So far this year, during the twelfth year of the Afghan War, 91 U.S. military personnel have given their lives in Afghanistan.

The Department of Defense reported 17 casualties in May and 18 in  June--the two deadliest months for U.S. troops in Afghanistan so far  this year. Typically, the summer months from June to September are the  deadliest for U.S. troops serving in Afghanistan.

Among the 91 U.S. casualties reported so far this year, all but 9 have been combat-related.

During the now 12-year-long Afghan War, the four deadliest years for U.S. troops were also the four years of Obama’s first term.

Total U.S. Combat Deaths in Afghanistan

2001: 5

2002: 30

2003: 31

2004: 49

2005: 94

2006: 87

2007: 111

2008: 151

2009: 303

2010: 497

2011: 494

2012: 294

2013: 91 (From Jan. 1, 2013 to Sept. 10, 2013)

Obama also has presided over the top five deadliest months of  the war, which include: August 2011, when there were 71 deaths; July  2010, when there were 65 deaths; June 2010, when there were 60 deaths;  October 2009, when there were 58 deaths; and August 2010, when there  were 55 deaths.

On Feb. 17, 2009, less than a month after taking office, President  Obama announced the deployment of 17,000 additional troops to  Afghanistan, fulfilling his campaign promise to shift U.S. military  forces to Afghanistan.

During a speech to cadets at the United States Military Academy  (USMA), at West Point, on Dec. 1, 2009, President Obama announced an  additional increase of 30,000 troops in Afghanistan. The president  stated that “it is in our vital interest” to send the additional troops.

The CNSNews.com database of U.S. casualties in Afghanistan is based  on official Department of Defense casualty announcements, International  Security Assistance Force (ISAF) information and media accounts.

With the exception of four U.S. military personnel who died in ships  supporting missions in Afghanistan and 12 who died in Pakistan, the  database does not include U.S. military personnel supporting Operation  Enduring Freedom in other areas of the world.

(Operation Enduring Freedom’s main battlefield is in Afghanistan but  it includes smaller military operations in the Philippines, the Horn of  Africa, Caribbean, and Trans Sahara, as well as support operations from  aircraft carriers and other U.S. vessels.)