As Israeli invasion forces begin their second month in Lebanon, there are signs that doubts and concerns are emerging among at least some of the troops about Israel's military role here.

According to two junior officers, opposition in Israel to the involvement here--especially large antiwar demonstrations there--is a major topic of conversation among the troops.

Some Israeli soldiers also seem to be sensitive about accusations that they are inflicting heavy civilian casualties on the Lebanese population. Some also reportedly are questioning whether this is really a "defensive" war as their leaders have told them.

However, there is no sign yet that antiwar sentiments in Israel or the troops' questions are affecting their discipline or ability to fight. And the soldiers appear heartened by the generally welcoming attitude so far of the Lebanese Christians, in whose zone the Israelis have installed themselves around Beirut.

A principal subject of debate among the soldiers, according to some who talked to reporters, is a recent demonstration in Tel Aviv originally estimated by Israeli radio at 50,000 to 70,000 people but estimated by other reporters on the scene to be far less.

"My men are arguing about it," said a 22-year-old first lieutenant guarding one of the crossing points into West Beirut. "They discuss it a lot."

He said he personally felt the organizers of such demonstrations "should wait until the fighting here is over. It's not nice to have that going on when you're fighting."

But, he said, "My opinion is we won't find a solution by war. Even if we take over their side West Beirut , it will leave many terrorists. They will start all over again somewhere else. You can't deny 5 million Palestinians. They have their own rights."

While he did not think Israeli troops were afraid to assault West Beirut, he said, there was concern about potentially high casualties.

"Every soldier thinks about it, especially if they see their friends get killed," he said.

"Of course the demonstrations have an impact on everybody," said a 31-year-old medical officer named Moses. "We have second thoughts and third thoughts about this war."

He added, "There's a feeling we're not fighting our war," a reference to the question of whether it is a defensive or offensive battle. Moses said he felt "ambivalent" about these questions and said he knew of no one who had refused to fight for political reasons.

"It's a luxury to think about these things" while the war is still going on, he said.

Moses, a reserve officer from Jerusalem, said he took offense at suggestions by opponents of the Israeli involvement here that the Israeli troops were waging an "inhumane" war. He said there were strict instructions not to shoot at anyone who surrendered and that wounded Palestinians were well treated, often being sent to hospitals in Israel for special care.

He said there had been cases of Israeli pilots refusing to bomb their targets because they were in populated urban areas. He said a friend returned from one mission with his bomb racks still fully loaded. He was supposed to hit a Palestine Liberation Organization headquarters, Moses said, but there were too many civilians in the streets around the target.

The pilot was not punished, Moses said, because standing instructions permit flyers to refrain from bombing if they do not have a clear target. Still, some pilots have deep regrets about the war, he said.

"We know pilots who have very bitter feelings," he said. "They are very exact, but if they miss it can be very cruel."

Moses expressed a suspicion reportedly shared by others that the Israeli Army is being dragged into an internal Lebanese conflict, not only fighting Palestinian guerrillas but propping up Lebanese Christian militiamen against their Moslem foes.

"The Christians want us to do all the fighting while they just watch," said Moses, a specialist in facial surgery. "I have no interest in having dead and wounded for these people. Let them solve their own problems."

A number of Israeli soldiers seemed curious about their enemy, frequently asking reporters if they had been to predominantly Moslem West Beirut, where an estimated 5,000 to 6,000 Palestinian guerrillas are trapped.

A young, soot-smeared Israeli soldier assigned to a 175-mm gun crew appeared eager to find out just how the other side feels about the war and whether they want to continue fighting.

"I want to stop," he said. "I want to go home."

According to an Israeli civilian covering the war for an Army magazine, many soldiers are "very angry" about a petition in the Israeli parliament to stop the "mass killing" in Lebanon.

However, he conceded that as in Israel, "everybody has a political opinion," and some soldiers support the Peace Now movement.

"Many think, 'What are we doing here? It's not our war,' " he said. "But they do their jobs because they receive orders."

He said that there still had been "no effect on morale."

Nevertheless, some unkind remarks are occasionally uttered about Israeli leaders, notably the architect of the invasion, Defense Minister Ariel Sharon.

Asked the other day if he was going into West Beirut, one soldier riding atop an armored personnel carrier yelled back, "Go ask the crazy fat man."