Washington ? The new U.S. Embassy in Baghdad will be the world’s largest and most expensive foreign mission, though it may not be large enough or secure enough to cope with the chaos in Iraq.

The Bush administration designed the 104-acre compound – set to open in September in what today is a war zone – to be an ultra-secure enclave. Yet it also hoped that downtown Baghdad would cease being a battleground when diplomats moved in.

Over the long term, depending on which way the seesaw of sectarian division and grinding warfare teeters, the massive city-within-a-city could prove too enormous for the job of managing diminished U.S. interests in Iraq.

The $592 million embassy occupies a chunk of prime real estate two-thirds the size of Washington’s National Mall, with desk space for about 1,000 people behind high, blast-resistant walls. The compound is a symbol both of how much the United States has invested in Iraq and how the circumstances of its involvement are changing.

The embassy is one of the few major projects the administration has undertaken in Iraq that is on schedule and within budget. Still, not all has gone according to plan.

The 21-building complex on the Tigris River was envisioned three years ago partly as a headquarters for the democratic expansion in the Middle East that President Bush identified as the organizing principle for foreign policy in his second term.

The complex quickly could become a white elephant if the U.S. scales back its presence and ambitions in Iraq. Although the U.S. probably will have forces in Iraq for years to come, it is not clear how much of the traditional work of diplomacy can proceed amid the violence and what the future holds for Iraq’s government.

“What you have is a situation in which they are building an embassy without really thinking about what its functions are,” said Edward Peck, a former top U.S. diplomat in Iraq.

“What kind of embassy is it when everybody lives inside and it’s blast-proof, and people are running around with helmets and crouching behind sandbags?”

The compound will have secure apartments for about 615 people.