Question: What military branch did 2013 All-Pac-12 star Pat Tillman enlist to in 2002?

Context paragraphs:
Pat Tillman: Patrick Daniel Tillman (November 6, 1976 – April 22, 2004) was a professional American football player in the National Football League (NFL) who left his sports career and enlisted in the United States Army in June 2002 in the aftermath of the September 11 attacks.  His service in Iraq and Afghanistan, and subsequent death, were the subject of much media attention.

Cyber force: A cyber force is a military branch devoted to cyberwarfare, cybersecurity and counter-cyberwarfare.  An example of a cyber force is the Norwegian Cyber Defence Force.  A Cyber force can be part of a military branch or a combined command.

Mike O'Callaghan–Pat Tillman Memorial Bridge: The Mike O'Callaghan–Pat Tillman Memorial Bridge is an arch bridge in the United States that spans the Colorado River between the states of Arizona and Nevada.  The bridge is located within the Lake Mead National Recreation Area approximately 30 mi southeast of Las Vegas, and carries U.S. Route 93 over the Colorado River.  Opened in 2010, it was the key component of the Hoover Dam Bypass project, which rerouted US 93 from its previous routing along the top of Hoover Dam and removed several hairpin turns and blind curves from the route.  It is jointly named for Mike O'Callaghan, Governor of Nevada from 1971–1979, and Pat Tillman, an American football player who left his career with the Arizona Cardinals to enlist in the United States Army and was later killed in Afghanistan by friendly fire.

Kevin Tillman: Kevin Tillman (born January 24, 1978) is a former U.S. Army soldier, an American anti–Iraq War activist, and former Minor League Baseball second baseman.  In 2002, Tillman left the Cleveland Indians organization after the September 11 attacks to enlist in the United States Army.  With his older brother, former National Football League star Pat Tillman, he completed the Ranger Indoctrination Program in 2002, and they were both assigned to 2nd Battalion—75th Ranger Regiment.  Kevin and Pat Tillman were deployed to the Middle East together as part of Operation Iraqi Freedom.  In 2004, Pat was killed in combat while in Spera, Afghanistan by friendly fire.

Where Men Win Glory: Where Men Win Glory: The Odyssey of Pat Tillman, a 2009 book written by Jon Krakauer, is a biography of Pat Tillman, an American football player who left his professional career and enlisted in the United States Army after the September 11 attacks.  To write the book, Krakauer drew heavily upon Tillman's journals, interviews with the Tillman family, "Boots On the Ground by Dusk: My Tribute to Pat Tillman" by Mary Tillman, and extensive research on the ground in Afghanistan.

2011 All-Pac-12 Conference football team: The 2011 All-Pac-12 Conference football team consists of American football players chosen by various organizations for All-Pac-12 Conference teams for the 2011 Pac-12 Conference football season.  The Oregon Ducks won the conference, defeating the UCLA Bruins 49–31 in the Pac-12 Championship game.  Oregon then beat the Big Ten champion Wisconsin Badgers in the Rose Bowl 45 to 38.  Stanford quarterback Andrew Luck was voted Pac-12 Offensive Player of the Year.  Cal linebacker Mychal Kendricks was voted Pat Tillman Pac-12 Defensive Player of the Year.

2014 All-Pac-12 Conference football team: The 2014 All-Pac-12 Conference football team consists of American football players chosen by various organizations for All-Pac-12 Conference teams for the 2014 Pac-12 Conference football season.  The Oregon Ducks won the conference, defeating the Arizona Wildcats 51 to 13 in the Pac-12 Championship game.  Oregon was then the national runner-up, in the College Football Playoff semifinal beating the ACC champion Florida States Seminoles 59 to 20; then losing to the Big Ten champion Ohio State Buckeyes 42 to 20 in the national championship game.  Oregon quarterback Marcus Mariota won the Heisman Trophy and was voted Pac-12 Offensive Player of the Year.  Arizona linebacker Scooby Wright III was voted Pat Tillman Pac-12 Defensive Player of the Year.

The Tillman Story: The Tillman Story is a 2010 American documentary film directed by Amir Bar-Lev.  The film is about the death of football player turned U.S. Army Ranger, Pat Tillman, in the war in Afghanistan, the coverup of the true circumstances of his death, and his family's struggle to unearth the truth.  It was nominated for the Grand Jury Prize at the 2010 Sundance Film Festival.  It was named 2010 Best Documentary by the San Francisco Film Critics Circle, the St. Louis Gateway Film Critics Association, and the Florida Film Critics Circle.  The film is narrated by Josh Brolin.

2013 All-Pac-12 Conference football team: The 2013 All-Pac-12 Conference football team consists of American football players chosen by various organizations for All-Pac-12 honors for the 2013 Pac-12 season.  The Stanford Cardinal won the conference, defeating the Arizona State Sun Devils 38 to 14 in the Pac-12 Championship game.  Stanford then lost to the Big Ten champion Michigan State Spartans in the Rose Bowl 20 to 14.  Arizona running back Ka'Deem Carey was voted Pac-12 Offensive Player of the Year.  Arizona State defensive tackle Will Sutton was voted Pat Tillman Pac-12 Defensive Player of the Year.

2012 All-Pac-12 Conference football team: The 2012 All-Pac-12 Conference football team consists of American football players chosen by various organizations for All-Pac-12 Conference teams for the 2012 Pac-12 Conference football season.  The Stanford Cardinal won the conference, defeating the UCLA Bruins 27–24 in the Pac-12 Championship game.  Stanford then beat the Big Ten champion Wisconsin Badgers in the Rose Bowl 20 to 14.  USC wide receiver Marqise Lee was voted Pac-12 Offensive Player of the Year.  Arizona State defensive tackle Will Sutton was voted Pat Tillman Pac-12 Defensive Player of the Year.

Supporting paragraphs: 2013 All-Pac-12 Conference football team Pat Tillman
Answer: United States Army