VINEYARD HAVEN, Mass. (AP) -- With his Oval Office speech Tuesday night, President Barack Obama will signal a shift in America's focus from the Iraq War to the war in Afghanistan, his spokesman said Thursday.

Administration officials have portrayed the 8 p.m. EDT speech as an important pivot point from a war that candidate Obama said should never have been fought to a conflict that President Obama sees as vital to the nation's security.

Previewing the speech as Obama vacationed on Martha's Vineyard, Deputy Press Secretary Bill Burton said Obama also wants to thank U.S. troops who've fought bravely in Iraq. With the formal U.S. combat mission at an end, troop strength in Iraq this week dropped below 50,000 for the first time since the 2003 U.S.-led invasion. Many of those troops will remain in a backup and training role.

Before his White House speech, Obama will fly to Fort Bliss in Texas on Tuesday to deliver his thanks in person to troops returned from Iraq.

Burton said the Oval Office address "commemorates an important milepost in American history." He said Obama will use the occasion to speak "directly with the American people about what our mission is in Afghanistan (and) the fact that more of our efforts and focus are now on fighting al-Qaida in Afghanistan."

Iraq has long been a partisan flashpoint, and the run-up to the fall congressional campaign continued the pattern.

Senate GOP leader Mitch McConnell said Obama had essentially adopted George W. Bush's strategy for gradually winding down the conflict.

McConnell said "the president (Obama) should be commended for ignoring his own campaign rhetoric."

While hailing Obama's Iraq drawdown, Burton denounced militants behind recent attacks on Iraqi security forces there. On Wednesday, a series of bombings and shootings left at least 56 dead.

"The reason for these attacks is people who don't want Iraq to flourish as a democracy," the spokesman said. "There are people who are trying to use fear and terror as a tactic to slow down what is not stoppable in that country."

Burton said Obama is confident the transition to Iraqi control "has been a successful one" and Iraqis are now capable of maintaining their own security.