The Turkish military threatened in April to cross into Iraq to fight the Kurdish Worker's Party, also known as the PKK, despite U.S. attempts to prevent a conflict in Iraq's most stable region.

The PKK is using Iraq's northern Kurdistan region as a staging ground to plan attacks against Turkish targets. Turkey has repeatedly asked U.S. forces and the Iraqi government to go after the PKK and has made multiple threats of intervening since the 2003 invasion.

"I have said to the Americans many times: Suppose there is a terrorist organization in Mexico attacking America. What would you do?" Turkey's Foreign Minister Abdullah Gul reportedly said in March.

Turkey did launch two major military operations against the PKK in Iraq in the 1990s, but also has continuously fought PKK within its own borders.

The PKK has a nearly 20-year history of violent clashes with Turkey in its fight for independence. Both the Kurdish separatists and the Turkish military have been accused of brutal attacks in which civilians were killed, according to human rights groups in the region.

The bloody legacy of the struggle makes the growing autonomy and success of Iraqi Kurds a threat to Turkey, which has a large Kurdish population and has long resisted and repressed the formation of a Kurdish state.

"Turkey wants a unified, stable Iraq on its borders and it sees that the current insurgency in Iraq and the Iraqi Kurds potentially breaking off as a threat near their borders," said Turkish expert Michael Gunter, a professor of political science at Tennessee Tech University.

Roughly 25 million to 30 million Kurds are spread through Turkey, Iraq, Iran and Syria, making them the largest ethnic group in the world without a state.

While there is no direct evidence that the Kurds of northern Iraq have been aiding the PKK, Iraqi Kurdish leader Massoud Barzani sparked the most recent tensions by threatening to get involved with Turkish Kurds if Ankara could not stay out of Iraqi domestic issues. The Iraqi Kurds also have not asked the PKK to leave the region.

Henri Barkey, a professor at Lehigh University and an expert on Turkey and the Kurds, said the Turkish general's proclamations about invading Iraq are mostly "bluster."

"They want [the United States] to do something about it, they really don't want to go into Iraq and they know they won't succeed," Barkey said.

Relations between the United States and Turkey have been strained since the invasion of Iraq. In 2003, Turkey's parliament rejected a U.S. request to use Turkey to launch a northern front in the Iraq war.

Then, as violence in southern Iraq continued to rage, the Kurds of Iraq's north became important allies for the U.S. forces, and U.S. officials held Kurdistan up as an example of the potential of a stable Iraq.

The perceived complacency by U.S. and Iraqi forces toward rebel elements in Kurdistan also angered Turkey. U.S. officials have placed that responsibility squarely on the shoulders of the Iraqi Kurds.

"The Kurdish leadership must do more to address this problem of terror and terrorism," David Satterfield, senior adviser to Secretary of State Condoleezza Rice, told Dubai-based satellite channel Al-Arabiya, in April.

But the PKK is seen by Kurds as a group that has fought for legitimate Kurdish rights, according to Gunter.

"If the Iraqi Kurds tried to help Turkey they would loose their own legitimacy," Gunter said.

The relationship between Turkey and Iraq has seen a mix of tension and cooperation since the U.S. invasion.

Turkey is by far the largest economic investor in northern Iraq, where it has funded the building of international airports, cement factories, supermarkets and other construction projects.

But Turkey takes issue with what it sees as Iraqi Kurds pushing for even more autonomy and power. The city of Kirkuk, which is the center of Iraq's oil industry, has become the eye of the storm.

Iraqi Kurds, claiming that Kirkuk is a historically Kurdish city that was stripped from them by Saddam, want a referendum by the end of the year to annex Kirkuk into Kurdistan.

The city's population is a mix of Arabs, Turkomans and a slight majority of Kurds. The non-Kurdish populations have voiced the desire to remain under federal control, and the Turkish government has weighed in on their side.

Turkey has claimed it has the legitimate right to look after the interests of the Turkomans, who are ethnic Turks. Turkish leaders also worry the acquisition of Kirkuk could give Kurdistan the economic base needed to split off into an independent state.

This fear is consuming to Turkey, said Barkey, even though it is not clear how many Turkish Kurds would favor joining an independent Kurdish state over the economic opportunities of Turkey.

However, Barkey said, the role of the Iraqi Kurds and the U.S. support for the region are significant.

"You do have a sense of burgeoning Kurdish identity throughout the region because of pride in what the Kurds of Iraq have accomplished," Barkey said. "They have been the most feisty, fought the hardest and suffered the longest."

-- By Talea Miller, Online NewsHour