AS the debate intensifies within the Obama administration over how to stabilize Afghanistan, one major problem is conspicuously missing from the discussion: the growing alienation of the country’s largest ethnic group, the Pashtun tribes, who make up an estimated 42 percent of the population of 33 million. One of the basic reasons many Pashtuns support the Taliban insurgency is that their historic rivals, ethnic Tajiks, hold most of the key levers of power in the government.

Tajiks constitute only about 24 percent of the population, yet they largely control the armed forces and the intelligence and secret police agencies that loom over the daily lives of the Pashtuns. Little wonder that in the run-up to Thursday’s presidential election, much of the Taliban propaganda has focused on the fact that President Hamid Karzai’s top running mate is a hated symbol of Tajik power: the former defense minister Muhammad Fahim.

Mr. Fahim and his allies have been entrenched in Kabul since American forces overthrew the Taliban in 2001 with the help of his Tajik militia, the Northern Alliance, which was based in the Panjshir valley north of the capital. A clique of these Tajik officers, known as the Panjshiris, took control of the key security posts with American backing, and they have been there ever since. Washington pushed Mr. Karzai for the presidency to give a Pashtun face to the regime, but he has been derided from the start by his fellow Pashtuns with a play on his name, “Panjshir-zai.”

“They get the dollars, and we get the bullets,” is the common refrain among Pashtuns critical of the government. “Dollars” refers to the economic enrichment of Tajiks and allied minority ethnic groups through an inside track on aid contracts. The “bullets” are the anti-Taliban airstrikes and ground operations in Pashtun areas in the south and east of the country.

While Mr. Karzai has tried to soften the image of Tajik domination by appointing Pashtuns to nominally important positions, much of the real power continues to rest with Tajiks. For example, he appointed a Pashtun, Abdul Rahim Wardak, to replace Mr. Fahim as defense minister — but a trusted Panjshiri, Bismillah Khan, remained an army chief of staff and kept fellow Tajiks as his top corps commanders and in other vital spots, including director of military intelligence, army inspector general and director of counternarcotics forces.

The National Security Directorate, which oversees the civilian and military secret police and intelligence agencies, is headed by a Northern Alliance veteran, Amrullah Saleh. Michael Semple, a former adviser to the European Union representative in Kabul, told me that Mr. Saleh had appointed “some credible Pashtun provincial directors” but that “the intelligence services are still basically seen as anti-Pashtun and pro-Northern Alliance because the power structure in the directorate is still clearly dominated by the original Northern Alliance group,” and above all because “they also have control of the prosecution, judicial and detention branches of the security services.”

The Obama administration is pinning its hopes for an eventual exit from Afghanistan on building an Afghan National Army capable of defeating the insurgency. But a recent study by the RAND Corporation for the Pentagon, noting a “surplus of Tajiks in the A.N.A. officer and NCO corps,” warned of the “challenge of achieving ethnic balance, given the difficulty of recruiting in the Pashtun area.” The main reasons it is difficult to recruit Pashtuns, one United Nations official recently said, are that “70 percent of the army’s battalion commanders are Tajiks” and that the Taliban intimidates the families of recruits. It doesn’t help that many of the army units sent to the Pashtun areas consist primarily of Tajiks who do not speak Pashto.

Pashtun kings ruled Afghanistan from its inception in 1747 until the overthrow of the monarchy in 1973. Initially limited to the Pashtun heartland in the south and east, the Afghan state gradually incorporated the neighboring Tajik, Hazara and Uzbek areas to the north and west.

It was understandable, then, that Pashtun leaders tried to make the last king, Zahir Shah, the president of the interim government that ruled from 2002 until the first presidential election in 2004. The king, revered by the Pashtuns, was to have limited powers, with Mr. Karzai, as prime minister, in day-to-day control. The Tajiks, however, objected, and on the eve of the national assembly that set up the interim government, the Bush administration’s special envoy, Zalmay Khalilzad, sided with the Tajiks and had a bitter 40-minute showdown with the king, who then withdrew his candidacy.

Pashtun nationalism alone does not explain the Taliban’s strength, which is fueled by drug money, Islamist fervor, corrupt warlords, hatred of the American occupation and the hidden hand of Pakistani intelligence agencies. But the psychological cement that holds the disparate Taliban factions together is opposition to Tajik dominance in Kabul. Until the power of the Panjshiris is curbed, no amount of American money or manpower will bring the insurgency to an end.