Margaret Warner reports.

　　MARGARET WARNER: This was the scene recently in a village not far from Mosul in Northern Iraq. Newly uploaded video purports to show Islamic State fighters doing battle with Iraqi Kurdish forces. Despite battlefield setbacks in Iraq and Syria, the militant group remains lethal.

　　How to fight ISIS has become a central theme in the 2016 U.S. presidential race.

　　DONALD TRUMP (R), Presidential Nominee: ISIS is honoring President Obama. He is the founder of ISIS. I would say the co-founder would be crooked Hillary Clinton.

　　MARGARET WARNER: That was last week. Yesterday, Republican nominee Donald Trump delivered a fuller anti-ISIS message in Youngstown, Ohio.

　　DONALD TRUMP: My administration will aggressively pursue joint and coalition military operations to crush and destroy ISIS, international cooperation to cut off their funding, expanded intelligence-sharing, and cyber-warfare to disrupt and disable a their propaganda and recruiting.

　　MARGARET WARNER: He also proclaimed that he would end what he called an era of nation-building, and would take harsh steps to stop ISIS from penetrating the United States.

　　DONALD TRUMP: The time is overdue to develop a new screening test for the threats we face today. I call it extreme vetting. I call it extreme, extreme vetting.

　　MARGARET WARNER: It would screen out those who sympathize with terror groups and those who have, in his words, any hostile attitude towards our country or its principles.

　　DONALD TRUMP: Those who do not believe in our Constitution.

　　MARGARET WARNER: In a Web video released last night, Democrat Hillary Clinton's campaign tried to turn Trump's own words against him, saying he would fail the test he'd set for immigrants.

　　Last November, Clinton said she would defeat ISIS by massing more U.S. ground troops against the group, though with limits.

　　HILLARY CLINTON (D), Presidential Nominee: And we should be honest about the fact that, to be successful, airstrikes will have to be combined with ground forces actually taking back more territory from ISIS. Like President Obama, I do not believe that we should again have 100,000 American troops in combat in the Middle East.

　　MARGARET WARNER: That fits with the picture of Clinton in a joint Washington Post/ProPublica report today about the early Obama administration debate over whether to fulfill his campaign pledge to pull out of Iraq altogether.

　　It notes that Clinton was — quote — "one of the most vocal advocates for a muscular U.S. presence in Iraq after the withdrawal deadline at the end of 2011." Clinton lost that argument, and all U.S. fighting forces left.

　　It's also been widely reported that, in 2013, Clinton and then CIA Director David Petraeus proposed arming and training the so-called moderate rebels in neighboring Syria, but that the president rejected it.

　　Those U.S.-backed rebels are still doing battle against Syrian President Bashar al-Assad, but with mixed results, in the brutal five-year old civil war that continues to this day.

　　For the "PBS NewsHour," I'm Margaret Warner.

　　HARI SREENIVASAN: So, what are the differences between how Donald Trump and Hillary Clinton would combat ISIS?

　　For that, we turn to Walid Phares, a foreign policy adviser to Donald Trump. And Wendy Sherman, she was undersecretary of state for political affairs while Hillary Clinton was secretary of state. She's now an outside adviser to the Hillary Clinton campaign.

　　Walid Phares, what's the key strategic difference that Donald Trump wants to make in the fight against ISIS that the Obama administration has not?

　　WALID PHARES, Foreign Policy Adviser, Trump Campaign: Well, first of all, very important to know that, between now and 2017, many things will change on the ground, and they will change for either Mrs. Clinton or Mr. Trump. So, anything we're projecting right now has to do with the moment.

　　There are major differences. First, in looking at the three battlefields, one is Iraq, Syria and, of course, Libya has to be dealt with. Three levels are important in terms of difference.

　　Level number one is who — what forces are going to be engaging ISIS on the ground? Is it Kurdish forces, the Iraqi army or others in Syria? And why do we ask this question? Because we don't want to end up with a sect controlling another sect on the ground, which will found the next war.

　　Second is also, who would take over after liberation from ISIS? Should it be the locals, national, the government, or a coalition of regional forces that would help them? And, thirdly, of course, what is the future of civil wars such as in Syria? Who will stay? Who will go?

　　And I think we have tremendous differences in how to go in, how to manage and, of course, the negotiations for the future.

　　HARI SREENIVASAN: So, Mr. Phares, one of the things that Donald Trump has said in a debate in March is that he's open to up to 20,000 to 30,000 U.S. troops being on the ground. Is that necessary? Is that the right course?

　　WALID PHARES: Mr. Trump made that statement. He may make other statements.

　　These are decisions that only when Mr. Trump is the president, hopefully, with his national security Cabinet, will decide upon the time. President Obama, for example, didn't want to send forces to the region after the withdrawal from Iraq. He had engaged in a warfare situation in Libya. He is sending forces.

　　So, these are national security decisions that would be decided once there is an evaluation of the situation on the ground. The American public in general has no appetite for sending tens of thousands, but each situation has a condition.

　　HARI SREENIVASAN: Ms. Sherman, Secretary Clinton has also called for ground troops, but in a limited capacity.

　　What do you see as the differences between what Mr. Trump is proposing and what Mrs. Clinton would carry out?

　　比较希拉里和特朗普对ISIS的应对方法

　　WENDY SHERMAN, Former State Department Official: Well, first of all, all of the things that Walid just outlined were not discussed in Mr. Trump's speech at all yesterday.

　　In fact, the strategy that Mr. Trump put on the table, other than the extreme vetting, is exactly what President Obama and Secretary Clinton have worked toward. That is an international coalition with local troops on the ground, having a very aggressive strategy in the cyberworld to stop the financial flows.

　　All of these are part of a multivector strategy that has been under way under President Obama for quite some time now and is actually having success.

　　Just today, Secretary of Defense Carter said that Syrian democracy forces had indeed taken back Manbij, which is a very key transit point, and now opens the way to ultimately getting to Raqqa, which ISIL has said is its centerpiece for a caliphate, which is disappearing on the ground in Syria.

　　There is a very complex environment in the Middle East. In that, Walid is correct. But it can't come without some knowledge and some background. And every day, we get a different message from Mr. Trump. I would like to know, does he still support torture, which is not the American way and doesn't bring results?

　　Does he still believe that we ought to be killing innocent civilians if there is a family of terrorists that have nothing to do with the terror? Is he someone who still believes in, as you pointed out, Hari, sending thousands and thousands of troops? Mr. Trump has been on all sides of that issue over the history of the last several years.

　　So it's very difficult to know whether Mr. Trump stands and whether he has an understanding of the complexity of the situation and the progress that's being made, but the progress that is still absolutely needed to protect our homeland and to make sure that Americans feel safe and secure.

　　HARI SREENIVASAN: One of the things that she said about extreme vetting, what does that mean? We have heard that there might be an ideological questionnaire. But if I'm a terrorist, wouldn't I just lie?

　　WALID PHARES: The final goal of this is to interdict jihadists from coming to the United States. Everything else could be reconstructed.

　　He needs to have the input of national security agencies. One of the problems with our analysts and our national security agencies over the past eight years have been encountering is that the ideological discourse that the jihadists have among themselves has been removed, removed from the analysts.

　　So it would be very difficult to be preemptive in the sense to understand when there is radicalization. This is something that our liberal democratic allies in France, in Britain and also in other countries and also in the Arab world have not done.

　　We have retreated from the ideological element. It's not that we are against one or the other ideology. But we need some indicators that these people are Salafis, are Takfiri, are jihadists, so that we can vet them.

　　Extreme vetting is not a physical extreme vetting. It's an intellectual exercise that would bring us back to where we should have been, understanding better the ideas that radicalizes these jihadists.

　　HARI SREENIVASAN: One of the things that Mr. Trump also said yesterday was that he was — the similarity that existed between Orlando and San Bernardino, the attacks, were that they were carried out by children and grandchildren of immigrants.

　　Is there a particular generation where Americans are patriotic enough where they wouldn't be — fall under the spell of ISIS?

　　WALID PHARES: What he meant by that was not actually a sociological interpretation of how these communities would work, because we also have immigrants and sons and daughters of immigrants from the Arab and Muslim war who fought in our armed forces and who died for America.

　　What he meant by that is, despite the fact of integration, the French are telling us the same thing, the Germans are telling us the same thing. So, integration is not the answer. It's basically deradicalization.

　　So, we want to make sure that this ideology doesn't go and thrust through the generations to a third one. It would be the same case for a neo-Nazi or an anti-Semite or a Bolshevik. It's not about a social problem. It's about an intellectual, ideological problem.

　　HARI SREENIVASAN: Wendy Sherman, one of the distinctions that I think exists between Hillary Clinton and President Obama is the institution of a no-fly zone over Syria.

　　President Obama, even as recent as the G20, said that that would be counterproductive. What would a President Clinton do in that case to make it work?

　　WENDY SHERMAN: First of all, Hari, in answer to your question about what extreme vetting is, quite frankly, I didn't understand Walid's answer.

　　This is not an intellectual exercise. This is about our immigration policies. And they are very strict, and the vetting is very tough. And our authorities are always looking at ways to make sure that we are as clear as we possibly can be.

　　As Walid himself knows — he's not a Muslim, but he came here himself in 1990, when he no longer felt personally safe in Lebanon because of his own history, which we could discuss at another time.

　　So, I don't quite understand yet what extreme vetting means, other than a nice sound bite on television.

　　To your point about a no-fly zone, Secretary Clinton has said that she wants to explore whatever alternative may deal with the really tragic humanitarian disaster which has played out in Syria. You know that there are literally millions of people who are now refugees.

　　There are millions of people who are internally displaced. The leader of Syria, Bashar al-Assad, has used starvation as a weapon of war, has used chemical weapons against his own people, has used chlorine gas probably against his own people.

　　And so the question is, in fact, how do we create some humanitarian safety for all of these millions of people who are really in a desperate, desperate situation? We have put enormous pressure on Turkey, on Jordan, on Iraq, and now on Europe, as migrants and refugees pour out of Syria looking for safety.

　　So, I applaud Secretary Clinton in wanting to explore every alternative, even knowing some of these are quite tough to do. And she will look very carefully to see what is doable. But we can't not try to see if there is an answer to this humanitarian tragedy.

　　HARI SREENIVASAN: All right, Wendy Sherman, Walid Phares, thank you both.

　　WALID PHARES: Thank you.

　　WENDY SHERMAN: Thank you.