THE CRITICAL scene in the movie "Gallipoli" comes when the kindly Australian colonel is summoned to his general's headquarters and told to prepare to go over the top of the trenches with his men the following morning -- an attack that the colonel knows is certain suicide. The general, safe and secure far from the scene where the fatal battle will occur, brushes aside the colonel's protests and orders him to make the attack.

The colonel obeys and the inevitable slaughter follows -- for no good reason, as it turns out. The British troops who were supposed to be part of the attack were drinking tea on the beach while Australians were being mowed down by the Turks.

I found myself thinking about "Gallipoli" this week while watching and reading about the inquiry into the Challenger disaster. In the world according to movies, issues are clear-cut in a way that they never are in life. In movies, we are blessed with the ability to divine the difference between the impossible and the glorious long-shot.

In life, we can never be sure -- so many penny philosophers remind us -- until we try. "Our doubts are traitors that make us lose the good we oft might win by daring to attempt," Shakespeare said in "Measure for Measure." Thoreau, too, cautions us against giving up before we have made the effort. "What everybody echoes or in silence passes by as true today may turn out to be falsehood tomorrow," he wrote in "Walden." "What old people say you cannot do you try and find you can."

According to the prevailing elan of the battlefield and the locker room, winners are the ones who think they can; losers are the ones who think they can't, or aren't sure. Winners revel in the opportunity; losers see only obstacles, problems and risks too dangerous to hazard the attempt.

Within hours after the Challenger shuttle disaster, President Reagan correctly and properly reminded us that this country was not settled by timid people. The pioneers who crossed the country were not the sort of men and women who turned back because the river's current was too swift or because Indians might attack. Tricky currents and hostile Indians were an inescapable part of a life that offered no certainties.

We understand that any effort to push back frontiers will claim casualties. Christopher Columbus knew the risks he was taking, presumably, just as surely as Lewis and Clark, the Wright Brothers, Chuck Yeager and John Glenn knew. And, presumably, they did not set out believing that they would fail in their missions, but that they would succeed. And believing that they would succeed surely helped them to do so.

We revel in stories of men and women who undertake perilous missions where the odds are heavily stacked against them and succeed. But we also know that men and women psych themselves up for success, and then fail miserably. Their attitudes may have been everything they should have been but they may not have been adequately trained, or they were poorly equipped; or perhaps their adversaries, whether human or nature itself, were simply too much for them. Someone has to be stronger in a contest.

Or perhaps they confused a "can-do" spirit with the notion that all of life is a movie that we can script at will, that somehow if we wish it hard enough, we can make it happen. "Can-do" says that I recognize the obstacles and risks here and that I'm willing to go ahead anyway. The life-as-movie view recognizes no natural limits; it says that nothing can stop me. That way is the road to folly and disaster.

Asserting -- even believing -- that the effort will be successful does not guarantee it. A positive attitude especially does not help when the one holding it is not participating in the mission. What made the Vietnam War stink in the nostrils of many of us -- even while we were under arms during its height -- was the ease with which those in the chain of command, from the president of the United States down through the Army and Marine brass drawing lines on maps, could affect that "can-do" attitude. Hundreds and thousands of young men may well have died thinking positively. Too often, they were betrayed.

The comedian Jonathan Winters used to do a brilliant sketch about the officer briefing his troops the night before battle, describing the fire they would come under on their way to their objective. "I had hoped to be with you," he says at the end, "but that will not be possible. However, I will be watching you from the bridge of the ship through heavy field glasses."

I am wondering if the astronauts who appeared so chipper on my television screen the morning of the fatal launch would have hesitated had they known what others did about the frailties of the rockets that were to carry them into orbit. I am wondering which of the NASA and Morton-Thiokol executives who were pushing for the launch -- despite the doubts and arguments of others against -- would have gladly suited up and occupied an eighth chair in that cabin that became a coffin.

Were I going into combat or on a perilous mission, I would want to be led by someone who not only believed in our objective and our chances for success, but loved life enough to avoid bravado and to defy bureaucratic bullies. If there are risks to be taken, let those giving the orders share the dangers.

What I was looking for in the late '60s and early '70s was a sense of honor and shame among those responsible for the useless maiming of American men in Vietnam. In Japan, in other times, those responsible for disgrace fell on their swords. We need nothing that extreme. It would be enough for those responsible to own up to it, as British Foreign Minister Lord Carrington did when Argentina proved him wrong by invading the Falklands, and he resigned. What we get at best is finger-pointing and excuses or, at worst, remorse for the victims -- who are labeled "heroes" -- and medals for meritorious service for those who are, in fact, to blame.

Wiser, less ambitious decision-makers would not need to be instructed again that life has limits and that the natural laws of the universe and the physical properties of this world don't change just because they stand in the way of our success. When we defy those laws or forget them, we are being reminded now, we pay a price.