YONGPYONG, South Korea -- Lt. Brad Harris kept his gas mask ready for use as he pointed to the gullies where he would expect North Korean tanks to be lying in wait for him.
He was commanding a 70-ton M1-A1 Abrams tank on field maneuvers, a bit south of the minefields and razor wire that mark the border of North and South Korea.
He and the 37,000 other U.S. troops in South Korea play a waiting game, standing by in case of a North Korean attack.
And this winter the border between the two Koreas, with its tank traps and minefields, with hundreds of thousands of enemy troops staring eyeball to eyeball, is more tense than usual.
The South Korean and North Korean governments normally agree on nothing, but these days both say there is a heightened danger that war will break out. Most security experts say that this is quite unlikely, but some are disquieted by recent military intelligence, such as the discovery that North Korea has transferred 110 military aircraft to the border area.
They are also uneasy because of the virtual black hole of intelligence about North Korea. Americans and South Koreans are not even entirely sure about such basic questions as who is running the country, and they say that this makes it harder than usual to predict what the North will do.
The most common view among experts is that Kim Jong Il, the army chief, has been in effective control since the death of his father, President Kim Il Sung, in 1994. But the posts of president and Communist Party chief remain vacant.
"It's not clear whether Kim Jong Il is controlling the military elite, or whether the military elite is controlling Kim Jong Il," said Cha Young Koo, a South Korean Defense Ministry planner.
The South Korean president, Kim Young Sam, has ordered his armed forces to be especially alert, and the South Korean army has raised its readiness one notch on its "watch scale." South Korean fighter aircraft are flying more sorties, and intelligence analysts are scrutinizing more data for worrying signs.
"Are we concerned?" asked Jim Coles, a spokesman for the 37,000 U.S. troops in South Korea. "Of course. We'd be fools not to be."
But Mr. Coles and others interviewed also emphasized that the indications were far from conclusive and that there was no sign a confrontation was imminent. Although tensions are rising, both sides have everything to lose by going to war, while they may each have a bit to gain politically by talking tough.
North Korea, for its part, denies any aggressive intentions. It warns that it is the "South Korean puppets and U.S. imperialist war maniacs" who are creating an extremely dangerous situation.
Almost everyone agrees that there is one excellent reason to take the North Koreans at their word when they deny any intention of invading: By most accounts, a North Korean attack would be suicidal.
Still, South Korean and American military officers say North Korea has repeatedly behaved in unpredictable or irrational ways, and they cite reasons for concern:
* Since last fall, North Korea has moved bombers and MiG-17 and IL-28 fighters to bases close to the border. North Korea has also moved long-range artillery, including 170-millimeter guns and 240-millimeter multiple rocket launchers, to the area near the border.
* Last fall, armed North Korean soldiers were caught infiltrating the South in two incidents, South Korean officials said. This was the first time in many years that the North was caught infiltrating soldiers, and it suggested the North might be trying to test the South's surveillance of the border.
* North Korea has reshuffled its military leadership, retiring elderly leaders and promoting younger generals from field armies. The new commanders are regarded as hard-liners, and the army appears to have gained power in the past couple of years.
Intelligence analysts are divided about the risks -- there is some counter-evidence: North Korea has cut back on winter training in the past few weeks, its air sorties have dropped sharply and it has been trying to improve relations with the United States.
Some analysts emphasize that South Korea's explosive economic growth -- 9 percent last year -- means that North Korea's military is losing ground to the South's.
"It's got to be obvious that they aren't going to win economically," said Col. John W. Reitz, a public affairs officer for U.S. forces in Korea. "So you've got to wonder if they're going to feel that they've got this great military machine and they've got to use it or lose it."
Some experts also worry that the North Korean leadership may even believe that armed clashes or a limited war might be in its pTC own interest. These analysts fear that the North Korean government might find it useful to have a crisis to blame its economic troubles on and divert domestic grumbling.
"Their actions may not be based on rational calculations, but on domestic political considerations," said Gen. Park Yong Ok, the assistant defense minister for policy. "That's why we're worrying about war."