BAGHDAD — Vice President Dick Cheney opened a new U.S. push for political unity in Iraq on an unannounced visit today, just ahead of the fifth anniversary of the U.S.-led invasion.

Cheney landed at Baghdad International Airport, then flew by helicopter into the heavily secured Green Zone for talks with Iraqi leaders and U.S. military and diplomatic officials. It was his third vice presidential trip to Iraq, where 160,000 American troops are deployed and the U.S. death toll is nearing 4,000.

For security reasons, Cheney officials divulged few details about the vice president's schedule, but said he was expected to make stops throughout the country, speak to troops and spending time with Prime Minister Nouri al-Maliki, U.S. Ambassador Ryan Crocker and Gen. David Petraeus, the top U.S. military commander in Iraq. Crocker and Petraeus are scheduled to travel to Washington next month to give a status report on the war.

Oman was scheduled to be the first stop on Cheney's 10-day trip to the Mideast, but on Sunday night, he left Air Force Two parked on a tarmac in England and boarded a C-17 for the flight to the Iraqi capital.

The future of Iraq will be discussed in his closed-door talks with leaders in Oman, Saudi Arabia, Israel, the Palestinian territory and Turkey. But Cheney's discussions at each stop also will touch on Iran's nuclear program and its desire for greater influence in the region, high oil prices and the pursuit of an Israeli-Palestinian peace deal that President Bush wants to see before he leaves office.

Cheney last visited Iraq in May 2007 before the president's buildup of more than 30,000 additional U.S. troops was in full gear. Bush dispatched the extra troops to reduce violence so Iraqi politicians could forge agreements that would bring minority Sunni Arabs into the government and weaken the insurgency.

Security has improved since last summer when the last of the five Army brigades arrived in Iraq to complete the military buildup, but Iraqi politicians are still in gridlock.

Cheney advisers say the vice president will highlight the reduction in violence and praise the fragile Iraqi government for passing some legislation aimed at national unity. In short, Cheney will compare and contrast Iraq before and after the increase in troops. He'll tell Iraqi leaders that they are on the right track and have made strides, but that now is the time to do more.

The war is entering its sixth year. It was on March 17, 2003, that Bush gave Saddam Hussein 48 hours to relinquish power. Three days later, U.S.-led forces began invading Iraq. The anniversary of the invasion is March 19 in the United States and March 20 in Iraq.