Washington ? The need for a big new military offensive in Iraq illustrates the difficulties still facing the United States after three troubled years of fighting. It comes as the Bush administration works to show critics at home and abroad it is on the job in Iraq and ready, if necessary, to confront new dangers from Iran.

Thursday’s assault against insurgent strongholds near Samarra by U.S. and Iraqi troops came as the administration issued a new report reaffirming its policy of pre-emptive strikes and directing attention to the dangers of a nuclear-armed Iran.

White House officials were quick to deny any connection between the assault, which the military billed as the biggest air operation since the March 2003 invasion, and the national security update.

Both came as the administration scrambled to turn around public opinion. Polls show eroding support for the war and slippage in the traditional GOP advantage on national security. President Bush has embarked on a series of speeches defending his Iraq policy.

In its 49-page report, “The National Security Strategy,” the White House said Iran topped the list of nations threatening the United States.

Some analysts suggested the report, in total, represented an attempt to change the subject from Iraq.

“It’s clear they are shifting their focus somewhat in terms of depicting Iran as the next great threat to the United States,” said Kurt Campbell, a former deputy assistant secretary of defense.

The new White House report accused Iran of supporting terrorists, threatening Israel and disrupting democratic reform in Iraq. In an opening statement, Bush cautioned that diplomacy to halt Tehran’s nuclear activities “must succeed if confrontation is to be avoided.”