(CNN) - In 2013, terrorists carried out the first major attack on U.S. soil since flying hijacked airplanes into the World Trade Center and Pentagon 12 years earlier.
The Boston Marathon bombing last April raised questions about why U.S. security agencies failed to prevent it.
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But as 2014 begins, the biggest national security debate focuses on government surveillance programs disclosed by classified leaker Edward Snowden.
A former National Security Agency contractor, Snowden made public details of several programs including how the government collects data on nearly every phone call made in the United States for possible use in terrorist investigations.
The expanded surveillance efforts emerged in response to the 9/11 attacks, and now Snowden's disclosures and the impact of technological advances have combined to start a transition in how the government combats terrorism.
President Barack Obama claims success in blunting the effectiveness of the core al Qaeda group that carried out the 2001 attacks.
However, al Qaeda affiliates in Iraq, Syria, Yemen and some African nations have grown in effectiveness and their ability to carry out attacks and even seize territory.
While Obama seeks to move away from the "war on terror" narrative that began under President George W. Bush, the administration continues to work on ways to change the post-9/11 congressional authorization for the use of military force that governs U.S.'s efforts against al Qaeda.
"I intend to engage Congress about the existing Authorization to Use Military Force, or AUMF, to determine how we can continue to fight terrorists without keeping America on a perpetual war-time footing," Obama said last May in a speech at the National Defense University.
"Unless we discipline our thinking and our actions, we may be drawn into more wars we don't need to fight, or continue to grant presidents unbound powers more suited for traditional armed conflicts between nation states," he added.
At the same time, the rise of the al Qaeda affiliates and their threat to the United States provide the Obama administration with arguments to defend the surveillance programs under the Patriot Act passed after 9/11.
However, public outrage at the collection of phone records known as metadata has forced Obama to propose modest changes to tighten oversight of NSA programs.
The changes mostly involve the program that collects phone metadata, which has come under legal challenge with different federal court rulings divided on its legality.
Some legislators call for more substantive changes than what Obama proposes.