(CNN) The killing of four American soldiers in Niger in October drew attention to the role of US troops in western Africa, where several terror networks roam freely.

The United States has enemies all around in the region. Niger shares a border with Mali, where an al Qaeda affiliate and other Islamist groups thrive in the vast desert. It also borders Libya, where ISIS and other extremists are regrouping , and Nigeria, where Boko Haram runs amok.

The Defense Department said ISIS-affiliated fighters ambushed the US soldiers on October 4.

On Thursday, the Pentagon released a summary of a months-long investigation into the soldiers' deaths, pointing to a series of failures and deficiencies, including a lack of adequate training, that contributed to the ambush.

However, the document claimed that "no single failure or deficiency was the sole reason for the events of 4 October."

Here are things to know about US operations in Niger:

American troops have been in Niger for years

The United States has previously acknowledged it has troops there. But it's never gone into much detail.

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In 2013, the White House announced that President Barack Obama had deployed 100 military personnel to Niger.

"This deployment will provide support for intelligence collection and will also facilitate intelligence sharing with French forces conducting operations in Mali, and with other partners in the region," Obama said in a letter to the House speaker.

Small groups of US special operations forces advise local troops as they battle Boko Haram and al Qaeda.

Niger is crucial to the war on terror

The country, along with Chad and Mali, serves as a bridge between north and sub-Saharan Africa, and all three are significant transit routes for local al Qaeda and ISIS affiliates. The terror groups use these routes to generate revenue that helps them recruit, expand and export attacks, according to US officials.

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ISIS also uses the transit routes to move fighters northward, where they gain easy access to Europe and the West. The terror group is attempting to infiltrate the gold mining industry in Niger so it can sell the precious metal on the black market and finance terrorism, according to one official.

Al Qaeda in the Islamic Maghreb -- the terror group's local affiliate -- operates along the border between Mali and Niger despite a French-led military counterterrorism operation. The US military says it largely plays a supporting role by providing intelligence, surveillance and reconnaissance assets in support of French forces.

"Niger is an important partner of ours, we have a deep relationship with them," said Lt. Gen. Kenneth McKenzie, the director of the Joint Staff.

The slain American soldiers -- Staff Sgts. Bryan Black, Dustin Wright and Jeremiah Johnson and Army Sgt. La David Johnson -- were part of a team advising and assisting Niger forces when they were attacked last year. Five Nigerien soldiers also died in the ambush.

Before the Pentagon document's release, Sen. Tim Kaine, D-Virginia, had told CNN on Wednesday that he believes the troops "were engaged in a mission that they were not authorized by law to participate in and that they were not trained to participate in. And that is a significant reason that they tragically lost their lives." Kaine sits on the Senate Armed Services and Foreign Relations committees.

The US works with allies in the region

The United States is not the only nation with a large presence in the region, McKenzie said.

France has about 5,000 forces in the area, he said. The French operation also involves forces from Germany, Mali, Niger and other countries.

"We have about 1,000 forces distributed over the Chad Basin, most of them in Niger, but not all of them," McKenzie said last year.

The Chad Basin area includes the nations that border Lake Chad: Nigeria, Niger, Cameroon and Chad.

Nigeria militant group Boko Haram, which has pledged allegiance to the Islamic State, is a threat to the nations in the Chad Basin.

The US has drones there, too

This year, the US Africa Command will launch its MQ-9 Reapers -- "hunter/killer" drones with advanced intelligence gathering capabilities -- from an air base just outside the city.

Agadez is more centrally located and will provide the US military with surveillance over a larger, more significant area.

A US Army civil affairs team worked on community efforts this year to build relationships and engage with people whose lives will be affected by the drone operations