A new era in network news may have begun last night as America moved into the second week of war in the Persian Gulf. With "CBS Evening News" now expanded to an hour, all three broadcast networks have hour-long evening newscasts airing on a regular basis for the first time in their histories.

For at least a decade, network news divisions have been advocating one-hour evening newscasts, but network affiliates, whose own local news programs are key sources of revenue and pride, fought the move. Now, with a huge international story dominating the news and with the networks struggling to compete with cable's newly dominant CNN, affiliate resistance to at least a temporarily expanded evening news has declined.

The networks are calling the newscasts "Special Editions" partly as a means of soothing affiliate nervousness. When CBS became the last network to adopt the hour format, anchor Dan Rather told viewers it was "a special one-hour war update edition."

In a memo sent to all of its affiliates yesterday, CBS said the hour news would continue "for as long as warranted to provide special expanded coverage of the fast-breaking events in the war with Iraq." The memo also advised affiliates they would have six minutes of local advertising time to sell in the second half-hour of the show; the other two networks have a similar arrangement.

In 1980, ABC changed the television landscape when it turned its nightly "America Held Hostage" reports on the Iranian hostage crisis into "Nightline," anchored by Ted Koppel. The Persian Gulf War could precipitate the same kind of sea change in network TV.

Tom Brokaw, anchor of "NBC Nightly News," which has been at the expanded length for a week, was asked yesterday if the special editions should be considered a dry run for a permanent hour-long news.

"Oh sure," Brokaw said. "I'd like it to be a wet run, a dry run, a run run, whatever you want to call it. It's always been a hope here to do an hour each night." NBC had planned to begin its hour-long news Thursday night, Jan. 17, but ended up staying on the air straight through prime time with continuous war coverage that night.

In 1982 the networks made a concerted tactical push for the hour format. "NBC Research Finds Considerable Interest in Expanded Network News," declared one press release. Affiliates at all three networks rebelled, however, and within a month, Van Gordon Sauter, then president of CBS News, announced that the plan was "at this stage, dead."

Rather said yesterday, shortly before beginning his evening newscast, that he would like a nightly hour news program too, but doesn't consider the current special editions to be testing the waters for that.

"Not that I know of, no," Rather said. "Would I like it to be that way? Actually, no. I'm in favor of doing a one-hour broadcast, but my preference would be at 10 p.m., across the board, in addition to the half-hour program.

"We are in a war situation now, with millions fighting and perhaps dying, and I don't want to think about anything except how best to cover the war," Rather said. "When we've won the war, when the men and women come home, then we can take a breath and resume our efforts to do a one-hour newscast on its own merits."

The maiden voyage of the one-hour "Evening News" unfortunately ran aground at the midpoint last night. In Washington and many other markets, the program opened bumblingly at 7 p.m., with a black screen, then a CBS News logo, then a black screen and then the logo back again, accompanied by audio of David Martin reporting from the Pentagon.

At one point "CBS News" was on the screen and Martin was saying, "You have to wonder about the authenticity of this English-language sign." Audio from a game show was also heard during the foul-up.

CBS News spokesman Tom Goodman said the flub was caused by "an electrical problem in a switcher in the control room in New York." Viewers of Washington's Channel 9, which elected not to carry the full hour of network news, were seeing a halting attempt to replay the 6:30 feed of the newscast on a half-hour delay. The problem was corrected after two long and messy minutes.

It was the second false start of the week for "CBS Evening News." On Tuesday night the opening of the program was disrupted when demonstrators from the AIDS activist group ACT-UP sneaked into the studio. This turned out not to be, as first thought, the only time such a disturbance occurred. A gay rights activist disrupted the show in 1973 when Walter Cronkite was the anchor, CBS sources said.

Even if stations love the hour-long newscasts during war coverage, it's by no means certain the format would become permanent. Oddly enough, opposition sounded strongest yesterday at ABC News, where "Nightline" was hatched. Sherrie Rollins, ABC News spokeswoman in New York, said a permanent one-hour newscast is "not likely" and that when ABC News President Roone Arledge was asked about it, "his answer was no."

Peter Jennings, anchor of "ABC World News Tonight," could not be reached for comment.

Tim Russert, NBC News Washington bureau chief, sounded optimistic about an hour-long newscast evolving out of the special editions. "We hope it's a noble experiment that will become permanent," Russert said. "We're going to make this newscast so good that it will be the affiliates' idea to keep it."

One key affiliate sounded mighty cautious, however. Bill Bolster, president of Multi-Media Broadcasting and chairman of the NBC affiliates' news committee, said from St. Louis that NBC President Robert C. Wright had assured him and other broadcast group representatives at a meeting in Chicago on Wednesday that "this is definitely not a plan to take a half-hour away from the affiliates."

And yet Bolster didn't sound adamantly opposed, either. "This is fraught with problems," he said. "The future of local television stations is in the news and information business, that's for sure. There's nothing more important to a television station than its news image. So this is a tough call."

Bolster said only about half of the NBC affiliates are carrying the full hour of "Nightly News," but that many of those stations are in major markets. As on ABC and CBS, the program is designed with a midway break so that stations can carry both half-hours or just the first.

Some stations are airing the second half-hour in so-called "prime access" time, between 7:30 and 8, when they normally run game shows like "Family Feud" or infotainment programs like "Entertainment Tonight."

Some of these stations are getting better ratings with the second half-hour of network news than they got with the game shows, Bolster said. But of course, there's a war on. That doesn't mean a permanent hour-long newscast is in great demand.

And long-term contracts with the distributors of the game shows would make it difficult for stations to clear time for expanded network news on a permanent basis.

Whether the viewing public wants more war news or less is a matter of great debate. Phil Donahue devoted his syndicated talk show to that subject yesterday. Most of those in Phil's studio audience seemed to think the coverage had been excessive.

Brokaw said he thinks the intensity of the coverage will abate. "You'll begin to see it now begin to crank down," he predicted. "We're not going to the Pentagon every time there's a briefing anymore."

One reason the networks have had so much extended coverage, in daytime, prime-time and late-night hours, is that the advertising market is very weak right now -- weak before the war began and even weaker since, with advertisers such as airlines, which face declining business, curtailing or dropping their commercials.

David F. Poltrack, CBS Inc. vice president, said the coverage is costing the networks less in lost advertising than it would during boom times.

"We are better able to accommodate coverage of the war because we are in a relatively soft advertising market," Poltrack said. "Our losses are not as great as they might be if this were taking place in a bullish market."

Networks have been able to move some advertising from preempted entertainment shows into their news specials, sources said, and have sold some new ads during the coverage, but at radically reduced prices. A 30-second spot on a major hit like "The Cosby Show" on NBC could cost an advertiser $250,000. The price for the same amount of ad time on a news special replacing that program could be as little as $50,000.

As for the hour-long evening news, the networks are proceeding cautiously. The quality of the wartime special editions could have a strong influence. "It's especially crucial that we show the affiliates that with or without a war, we can do a compelling job of doing a one-hour newscast," said Brokaw.

"We're going to have to take it one step at a time," said NBC Vice President Betty Hudson. "We'll see how this chapter goes, and then, on to the next."