War has broken out between MSNBC and Fox News Channel.

Fox News star Geraldo Rivera lobbed the first grenade on Monday, when he appeared on Fox News Channel to deny reports that the Pentagon was expelling him from Iraq for revealing sensitive information.

"It sounds to me like some rats at my former network, NBC, are spreading lies about me . . . trying to stab me in the back. . . . MSNBC is so pathetic a cable news network they have to do anything they can to attract attention," he said, surrounded by members of the 101st Airborne.

MSNBC fired back with a segment on Tuesday about how Geraldo had become part of the Iraq war story, with anchor Lester Holt interviewing TV Guide senior editor Max Robbins.

Immediately after that segment, MSNBC cut to a promo that featured a waving American flag and the following printed message:

"MSNBC makes a pledge to all Americans. We will keep you informed by providing valuable, objective reporting. We will not compromise military security or jeopardize a single American life. We take both duties seriously. Let us know how we're doing." It included the network's e-mail address in the ad.

Yesterday morning, both sides continued their assault, using the crawls at the bottom of the screen.

MSNBC, in its crawl, told its viewers that the Pentagon had tossed Geraldo.

Actually, the Pentagon on Monday initially had said that Geraldo was being removed because he had "compromis[ed] tactical information." For broadcast, the FNC correspondent had sketched in the sand the location of various coalition forces and described a plan by one unit to join the surrounding of Najaf. He gave "real-time information about a unit's location, their mission and their pending activity, which would clearly aid the enemy," according to Pentagon spokesman Bryan Whitman. But, The Post's Howard Kurtz reported, after FNC Chairman Roger Ailes called a Defense Department official, the Pentagon amended that, saying the situation was under review. By yesterday the Pentagon's official position, Whitman told The TV Column, was that Fox News Channel had "pulled [Rivera] out of the theater," and that Fox News "completely understood our concerns and they took the matter seriously."

Meanwhile, Fox News Channel, in its crawl, reminded viewers that MSNBC had sacked Arnett on Monday in response to what FNC described as a public outcry.

MSNBC and its parent NBC fired Arnett over an interview with state-owned Iraqi TV, in which he said that America's "first war plan has failed because of Iraqi resistance" and that "our reports about civilian casualties here, about the resistance of the Iraqi forces . . . help those who oppose the war." That firing was a bit odd, given that MSNBC and NBC had been insisting all along that they never actually hired Arnett, who was officially over there in his capacity as a National Geographic TV employee.

At some point yesterday, FNC asked MSNBC to correct its crawl to say Geraldo had voluntarily left Iraq. FNC's position on the Geraldo affair is that he volunteered to return to Kuwait "after learning of concerns that he may have inadvertently violated the rules governing embedded journalists." MSNBC refused to make the change. So yesterday afternoon, around 3:30 p.m., FNC brought out its heavy artillery: a sensational 15-second ad blasting the network it had until that moment steadfastly referred to as "irrelevant."

"He spoke out against America's armed forces," roared the ad, which featured footage of Arnett giving his career-busting interview to Saddam Hussein's TV network.

"He said America's war against terrorism had failed. He even vilified America's leadership.

"And he worked for MSNBC.

"Ask yourself," the ad continued, "is this America's news channel?" (FNC apparently did not get the memo that MSNBC had dumped its short-lived "America's NewsChannel" brand in favor of "NBC News on Cable 24/7.")

The ad ends with FNC's usual sales pitch: We report, you decide, fair, balanced, blah, blah, blah.

Yesterday we contacted both sides to see if they couldn't try diplomacy first.

"It's interesting that they now find us relevant, since as recently as just a few weeks ago they deemed us irrelevant," MSNBC spokesman Jeremy Gaines told us. "I find it outrageous that they'd run this promo while they continue to employ Geraldo Rivera."

MSNBC also says the promo that ran following its Geraldo segment had been produced during the war in Afghanistan. Who knew news channels kept promos in the can so long.

Over at FNC, the response was no longer that "MSNBC is irrelevant" -- which was some progress, since that line was getting really stale.

"MSNBC should be more concerned about being an embarrassment to GE than [about] who we employ," the network said.

In their spare time yesterday, both news networks covered the carnage in Iraq.

Fox News Channel turned Peter Arnett's Iraqi interview into a promo ad.