The estimated 25-35 million ethnic Kurds spread across Turkey, Iraq, Syria and Iran are at the forefront of multiple conflicts reshaping the Middle East.  In Syria and Iraq, US-backed Kurdish forces have led the fight against the so-called "Islamic State" (IS).

However, "the Kurds" are riven by intra-Kurdish rivalries both within their respective states and across greater Kurdistan. As the United States backs Syrian and Iraqi Kurds, it has found itself in the middle of these rivalries and at odds with NATO ally Turkey.

The main intra-Kurdish fault line is between the Kurdistan Workers' Party (PKK) and its affiliates (see below) and the Kurdistan Democratic Party (KDP) led by Masoud Barzani, the president of the autonomous Kurdistan Regional Government of Iraq (KRG).

A divided Kurdish quasi-state

The KRG has many characteristics of a state -- an executive, legislature, judiciary and security forces -- all recognized under the Iraqi constitution's federalist structure. The United States, as well as European states including Germany, provide assistance to their longtime Iraqi Kurdish allies.

However, the Iraqi Kurdish army, known as peshmerga, or "those who face death," are not united under the same command even though they cooperate. Barzani's KDP and its main political rival with Iraqi Kurdistan, the Patriotic Union of Kurdistan (PUK), each have separate peshmerga forces.

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The PUK is closer to the PKK, the Iraqi central government and Iran. These rivalries play out in Syria and with Turkey, which is close to Barzani and his KDP.

US-backed Syrian Democratic Forces

In Syria, the United States backs the Syrian Democratic Forces (SDF) with weapons, airstrikes and about 900 Special Forces. Considered the best fighters against IS, the SDF is a roughly 50,000 strong force composed of Kurdish, Arab, Turkmen and Christian militia. It was formed in 2015 at US prodding in part to address Turkey's concerns over the dominance of the Syrian Kurdish People's Protection Units (YPG). 

The YPG  and the all-female Women's Protection Units (YPJ) are the armed wings of the Democratic Union Party (PYD), a left-leaning Kurdish political party in Syria. Together they make up about half of the SDF.

Kurdistan Communities Union

The PYD, in turn, is a part of the Kurdistan Communities Union (KCK), a pan-national umbrella political group founded by the PKK in 2005. Alongside the PYD, the KCK comprises the PKK, the Iranian branch Party for a Free Life in Kurdistan (PJAK) and the much smaller Iraqi affiliate, Kurdistan Democratic Solution Party (PCDK).

The KCK and its subset political parties are composed of various political, social and military subunits. They subscribe to the ideology of PKK leader Abdullah Ocalan, who has been in a Turkish prison since his capture in 1999.

Read more: Kurds in Germany protest against Erdogan, for Ocalan's freedom

Ocalan continues to be the PKK and the KCK's nominal head. However, the de-facto leader of KCK is its co-chair, Cemil Bayik, a top leader within the PKK.

The PKK has carried out a nearly four-decade long armed struggle against the Turkish state resulting in the death of about 40,000 people. Turkey, the United States and European Union consider the PKK a terrorist organization.

Turkey considers the YPG/PYD, as well as the SDF terrorist organizations for their ties to the PKK. This view stems in part from the fact that from the 1980s to late 1990s, the PKK and Ocalan operated out of Syria and Lebanon with the support of former Syrian President Hafiz Assad.

Syria kicked out the PKK in 1998 after Turkey threatened to invade, but then essentially handed over parts of northern Syria to the PYD shortly after the onset of the Syria civil war in 2011.

PKK under a different name? 

The PKK and PYD deny that they have organic organizational ties. The PKK and PYD say they have a different substructure, command and ultimately different goals in their respective countries, Turkey and Syria, given the different political situation in each with regards to the Kurds.

Unlike the PKK, which primarily fights the Turkish state, the PYD/YPG is focused on fighting IS and on occasion Turkish-backed Syria rebel groups. The PYD/YPG has not sided with Assad; in fact, the group has a tacit military understanding not to engage in combat with Syrian government forces. It also has not aligned with either Islamist rebel factions or Turkish-backed opposition, saying it has no designs on Turkey and wants to avoid conflict.

But the YPG counts hundreds of Turkish Kurds within its ranks, including PKK fighters who transferred to the fight in Syria. Before the Syrian civil war, the PKK drew about a quarter of its fighters from Syria, raising further questions over its links to the YPG.

Meanwhile, the United States has said it sees enough difference between the PYD and terrorist-categorized PKK to back the YPG and SDF units fighting in Syria. And as that relationship has grown over the past two plus years, the PYD/YPG has sought to publically distance itself from the PKK.

What binds the PKK and PYD, they say, is an adherence to Ocalan's Marxist-Leninist ideology and a shared desire to beat back jihadist forces. Ocalanism incorporates women's rights, human rights, environmentalism, communalism and ''democratic autonomy,'' a grassroots form of federal governance viewed by its followers as a model for democracy in Middle East.

This political model contrasts with that in Iraqi Kurdistan led by Barzani. There, the system is based on family and tribal ties, crony capitalism and patron-client relationships.

Facts on the ground

Off the battlefield, the PYD has set up an autonomous political structure based on Ocalan's ideas in areas under its control in northern Syria, known as Rojava. By creating facts on the ground, the PYD hopes to bolster Kurdish political claims in any future settlement in Syria.

Turkey fears Syrian Kurdish gains will embolden its own Kurdish population and create a PKK statelet on its southern border. This has created strains in Ankara's relations with Washington, including setting up the prospect that Turkey could clash directly with the United States in one of the many attacks it has carried out against the YPG.

A sustained conflict between the SDF/YPG and Turkey would undermine a key US goal, namely defeating IS and rooting it out of its self-declared capital Raqqa.

The PYD's detractors, including other smaller Syrian Kurdish parties, accuse it of monopolizing power and repressing dissent. They also accuse it of allying with the Assad regime.

As a result, Barzani's KDP has supported other Syrian Kurdish factions and, similar to Turkey, implemented a border embargo over PYD controlled areas, fueling intra-Kurdish tensions.