The success of the U.S. surge in Iraq was due, in part, to the 100,000 Sunni tribesmen hired by the U.S. Army to secure their communities. The continuing security of Iraq, at least in the short-term, may depend on their continued service. Unbelievably, that is now being threatened by the Iraqi government.

On Tuesday, the Pentagon expressed concern that the Iraq government won't put the fighters on its payroll in the transition of security operations to Iraqi control, which officially began this week.

It would be a major mistake.

Managing the transition from American-led and financed tribal security operations to Iraqi control is the next step in shifting security responsibilities to Iraqi shoulders. But continuing religious and tribal differences make the transition a tricky one. The Sunni fighters found American forces trustworthy allies. Can they find a similar mutual trust with the Shia leadership?

Doubts about the ability of the two sides to quickly develop a satisfactory relationship is a major reason why the Pentagon on Wednesday announced plans for sending additional forces to Iraq next year. The reinforcements, if needed, would maintain U.S. troop strength in Iraq at the present level of about 152,000 through 2009.

The Pentagon orders may simply be precautionary. It is prudent to make sure units are prepared to deploy on schedule in the middle of next year, whether they will actually be needed or not. It is wise to recognize that the next president may be faced with a big decision if continued ethnic friction in Iraq threatens the gains of the past 18 months.

Commanders in Iraq say the security situation remains fragile and reversible. A major worry is the possibility that Sunni tribesmen will turn against the government if they feel it does not respect and support their role in restoring local security. "The slow pace of transition" to Shia control of armed groups of Sunni militia "is a concern," said a Pentagon report to Congress on Tuesday.

The worst situation would occur if the U.S. walked away from Iraq before Sunni tribesmen were integrated into a new Iraqi security structure. Recall how Congress cut support for Afghanistan once the Soviet Union was forced to withdraw. That decision effectively surrendered Afghanistan to the fanatical Taliban and al-Qaida. The U.S. must not make the same mistake in Iraq.