The GSG9, which stands for Grenzschutzgruppe 9 (Border Protection Group 9), was set up in 1972 after regular German police failed to rescue Israeli hostages kidnapped by Palestinian terrorists at the Munich Olympics. Its formation was controversial, with some politicians in Germany feeling the group was reminiscent of the notorious Nazi SS.

The GSG9's very first mission, called "Operation Fire Magic," established its high reputation. After Palestinian terrorists hijacked a Lufthansa plane in 1977, the GSG 9 managed to rescue passengers in a seven-minute operation in Mogadishu. A GSG 9 member and a flight attendant were injured, while three of four hijackers were killed. Sadly, the pilot was killed before the operation took place.

Ulrich Wegener, who was a founding member of GSG 9, received an Order of Merit from the German government after the successful mission. Wegener, who became known as the "Hero of Mogadishu," died on December 28, 2017, at the age of 88. He was always uncomfortable with his popular title, saying recently: "We did the work together."

The GSG 9 goes into action in hostage situations, in cases of terrorism and to undertake bomb disposal. But it is also deployed to secure locations, as here ahead of the 2007 G8 summit in the northern resort town of Heiligendamm.

Most of the GSG 9's missions are confidential, but it is said to have participated in more than 1,900 operations since being founded. It is currently based in the western town of Sankt Augustin, near Germany's former capital, Bonn.

The GSG 9 members undergo rigorous training for all eventualities. Here, they practice dealing with an attack by armed terrorists on a railway station. Plans are now underway to expand the unit by a third and give it another headquarters in the capital, Berlin. Although the number of members is kept a secret, media currently put it at around 400.

The "Islamic State" (IS) — also known as ISIL, ISIS and Daesh — is an al-Qaida splinter group with a militant Sunni Islamist ideology. It emerged in the aftermath of the US-led invasion of Iraq in 2003 and is led by Abu Bakr al-Baghdadi. Their goal is to create a worldwide "caliphate." It gained worldwide notoriety in 2014 after a blitzkrieg military campaign that resulted in the capture of Mosul.

IS is believed to be operational in more than a dozen countries across the world. It controls territories in Iraq and Syria. However, the group has lost much of the territory it controlled in Iraq and Syria at the height of its expansion in 2014.

The US leads an international coalition of more than 50 countries, including several Arab nations. Russia, Iran and its Lebanese Shiite ally Hezbollah, which all support the Syrian government, also fight IS. Regional forces such as the Kurdish peshmerga (above) and US-backed Syrian Kurdish fighters, fight IS on the ground. The Iraqi army and militia have pushed IS from large parts of the country.

One of IS' main sources of income has been oil and gas. At one point, it controlled an estimated one-third of Syria's oil production. However, US-led airstrikes deliberately targeted oil resources and the Syrian government as well as US-backed Syrian Kurdish fighters have retaken most oil wells. Other means of income include taxes, ransom, selling looted antiquities and extortion.

IS has claimed responsibility for numerous terrorist attacks across the globe. The militant group has targeted capitals across the EU, including Berlin, Brussels and Paris. IS leaders have encouraged so-called "lone wolf" attacks, whereby individuals who support IS carry out terrorist acts without the direct involvement of the group.

The group uses various tactics to expand its power. IS fighters have looted and destroyed historical artifacts in Syria and Iraq in an attempt at "cultural cleansing." The group has also enslaved thousands of women from religious minority groups, including Yazidis. IS also uses a sophisticated social network to distribute propaganda and recruit sympathizers.

IS has further exacerbated the ongoing Syrian conflict. Millions of Syrians and Iraqis have fled their homes, many traveling to Europe in pursuit of refuge. Although it has lost all of its strongholds, the militant group has left extraordinary destruction in its wake. Areas affected by the militant group's rule will likely take years to rebuild.