What you need to know about Incirlik air base:

Over the past year, the Incirlik air base in Turkey has become a point of contention between Germany and Turkey.

With roughly 250 German troops stationed at the base, it marks a strategic outpost for Germany's military operations within NATO and the anti-"Islamic State" (IS) alliance.

DW looks at the importance of the Turkish base in the NATO alliance and how it became a contested issue in German-Turkish relations.

Why is the Incirlik air base important?

Constructed in 1955 with US assistance, the facility was NATO's most important "southeast wing" air base during the Cold War. For over 60 years, the base served as a deterrent to possible Soviet expansion and proved valuable in handling various crises in the Middle East.

During the First Gulf War, Incirlik served as the origin point from which coalition forces flew combat missions over Northern Iraq. Later, US-led coalition forces used the base for missions in Afghanistan as part of Operation Enduring Freedom.

In 2014, IS took over large swathes of territory in Syria and Iraq, prompting an allied response. When Turkey joined the anti-IS coalition in late 2014, it provided the Incirlik air base as a launch pad for combat operations against the militant group. Incirlik offers a strategic location for such combat missions due to its proximity to IS' operating locations.

In January 2016, Turkey and Germany struck a deal that allowed Berlin to deploy several reconnaissance aircraft to Incirlik from Hamburg to take part in the anti-IS mission. However, the planes do not directly engage in combat against IS.

What weapon systems are present at the base?

The US 39th Air Base Wing comprises the bulwark of the forces at the base. Alongside A-10 aircraft that provide close air support to ground forces, the US has stationed F-15 and F-16 tactical fighters at the base.

Incirlik periodically hosts AWACS reconnaissance aircraft and surface-to-air missile defense systems. Apart from Turkish weaponry, Incirlik reportedly hosts nuclear warheads.

Although never confirmed by NATO, US or Turkish officials, there are roughly 50 B61 type nuclear warheads that have been stationed in the base since the Cold War, according to the Federation of American Scientists.

Which countries operate out of Incirlik?

Excluding Turkish forces, the US has the most troops deployed at the Incirlik air base, with around 1,500 US military personnel on the ground, followed by the UK.

Germany reportedly has about 260 troops stationed at the base that play a support role for the deployed aircrafts.

The Netherlands, Spain, Denmark, Czech Republic, and non-NATO states Saudi Arabia and Qatar also have deployed troops to the base to take part in anti-IS operations.

Why is Incirlik a problem between Germany and Turkey?

As fellow NATO members since the 1950s, the two military allies have a decades-long relationship owing to their high-level of economic and socio-cultural links. However, back-and-forth tensions over Incirlik air base form part of a series of political spats that have arisen between the two countries over the past year.

In 2016, Turkey denied German lawmakers access to the air base to after the Bundestag, Germany's parliament, passed a parliamentary bill classifying the mass killing of Armenians by Ottoman forces in 1915 as "genocide." However, Ankara later reversed its decision and provided access to the parliamentarians.

In May, Turkey once again blocked German lawmakers from accessing the base over Berlin's decision to grant asylum to Turkish military personnel accused of participating in attempted coup centered on the overthrow of Erdogan in July 2016.

In response to Turkey's decision, German Chancellor Angela Merkel said authorities in Berlin will work on smoothing out the issues between the two military allies. However, she noted that Berlin is assessing alternatives to host the German deployment for the anti-IS coalition currently stationed at Incirlik. Merkel named Jordan as one of the possible alternatives under consideration.

What are Germany's options for relocating troops at Incirlik?

Germany's deployment at the Incirlik air base forms part of the anti-IS coalition's reconnaissance and refueling mission.

The German mission at Incirlik also provides extensive coordination and logistical cooperation with other NATO member states.

However, it is unclear howa decision to relocate German troops from Incirlik would impact the anti-IS mission and, even more so, further cooperation between Turkey and Germany as NATO allies.

Aside from Jordan, Cyprus and Kuwait are also being considered as possible alternatives to Incirlik, according to reports.

Although Jordan and Kuwait are not members of the NATO, they are partners of the alliance and participate in the fight against IS, making them suitable alternatives to Incirlik.