In an interview with the Washington Examiner, Simes identified the questions he hopes Trump will answer and which positions the GOP front-runner should clarify.

Nuclear proliferation

Trump turned heads last month when he repeatedly suggested that, as president, he would consider allowing Japan and South Korea to develop their own nuclear weapons so the U.S. could ease its defense commitment to both countries.

“You have so many countries already — China, Pakistan, Russia — that have them,” the billionaire told Wisconsin voters. “Wouldn’t you rather, in a certain sense, have Japan have nuclear weapons when North Korea has nuclear weapons?”

His comments sparked outrage among proponents of nuclear disarmament and national security experts who fear that arming Japan and South Korea would jeopardize the United States’ relationship with its Mideast allies.

“This is a matter that requires a serious and specific conversation and the very fact that Mr. Trump has mentioned it, that in my view is clearly very provocative,” Simes told the Examiner. “It’s vintage Trump and we need to understand, what does he mean by that.”

“I’m not sure at this point that he quite knows himself,” Simes added.

The U.S. and free trade

Like immigration, Trump has made U.S. trade policy a focal point of his campaign. He’s criticized his opponents’ support of the North American Free Trade Agreement (NAFTA), lambasted the Trans-Pacific Partnership (TPP) trade deal and urged the U.S. to impose substantial tariffs on China and Mexico, who are “taking our jobs.”

Many questions have been raised about the monumental impact Trump’s trade policies could have on U.S. industries and the economy and how he would get congressional approval for his plans to impose tariffs on foreign exports — questions that Trump himself has yet to answer.

“It will be interesting to hear why Mr. Trump thinks he would be able to push China so effectively without retaliation,” Simes said. “He has to make the case. He has to explain it.

He continued, “It is very important for [Trump] to at least describe his fundamental approach to free trade. He’s said he’s for free trade on a number of occasions, but in other instances he sounded [like] a real protectionist. ”

The Islamic State and U.S.–Russian relations

Trump’s proclaimed tactic to defeat Islamic State militants is to “knock the hell out of them,” bomb their oil fields and “take out their families.” The billionaire has also proposed barring Muslims from entering the U.S. as a measure to protect Americans against domestic terrorist attacks similar to the shooting in San Bernardino, Calif., last fall.

Simes told the Examiner he has “considerable concerns” about Trump’s plan to temporarily ban non-American Muslims from entering our country and believes the Republican front-runner should better articulate the type of legislation he wishes Congress would pass to protect the U.S. against immigrants from high-risk countries.

“What concerns me is when you deal with issues so explosive and so … painful to many people, you cannot deal with [them] with an axe,” he explained. “You have to deal with it with a scalpel. He has to demonstrate an attention to detail and sensitivity to the feelings of those on the receiving end of his statements.

While discussing U.S. efforts to combat radical Islamic extremism in an interview with Fox News last October, Trump opined that America can no longer be the “policeman of the world.” Trump has since promised to improve U.S. relations with Russia and other countries, if he’s elected president, in order to compel them to continue their own efforts to defeat the Islamic State — as controversial as they may be.

“We can use them to knock out ISIS with us so that maybe we don’t always have to pay for it,” Trump said of Russia during a GOP debate in November. “Let them drop some of their bombs that cost $1 million apiece, let them use some of their weapons that cost billions of dollars.”

“I think it’s very important to realize that we have a very difficult relationship with Russia today,” Simes said. “And Mr. Trump, in this instance, he is speaking like a businessman. He’s looking at countries like he would look at companies. Some may be a rival, some may be a partner.

But Simes said that if Trump intends to negotiate with Russian President Vladimir Putin, he needs to demonstrate the he can “control his temper.”

“A lot of people are not so much concerned with his views but they are concerned with how he reacts in situations of adversity,” Simes said. “I think it’s very unrealistic to think that even if Mr. Trump, or any American president, gives his best effort, that he will find that Putin is a willing collaborator.”

“That simply is not going to happen. So the question is, what would Mr. Trump do when Putin tries to cross him, as he probably will. Will he call him a ‘Lying lot?'” he added.

NATO

During a recent interview on ABC’s “This Week,” Trump called NATO “obsolete” and claimed the Cold War-era alliance has become disproportionately expensive for the United States to belong to.

Pentagon officials disagreed, describing NATO as “as relevant as ever;” Trump’s Republican rival Ted Cruz accused him of wanting to “withdraw from the world and abandon our allies;” and NATO Secretary-General Jens Stoltenberg said the alliance represents the “cornerstone” of cooperation between the U.S., Canada, and their European allies in combating global terrorism.

Simes described Trump’s suggestion that other NATO countries shoulder more of the cost as “perfectly appropriate,” but said it is “very important for Mr. Trump to explain how he’s going to go about this modification of NATO.”

“Would he just tell allies, ‘Boys, girls, you are not going to get our money unless you pay more,’ or would he engage in creative diplomacy [and] explain to them that we remain committed to their security, but there should be a game changer?” Simes said.

Building the wall

Perhaps the greatest promise Trump has made to voters this election cycle is his repeated assurance that he will build a “beautiful” wall along the Southern border and persuade Mexico to pay for it. Such a wall would prevent illegal immigrants from entering the U.S. where they are “taking our jobs” and “killing us,” Trump has claimed.

Altogether, Trump’s plans for immigration reform, including his border wall, would cost the U.S. an estimated $166 billion, Politico reported last August. The wall alone could range from $5 billion, a calculation based on figures provided in a 2009 Government Accountability Office report about the cost of border fencing, to the $8 billion price tag Trump himself has provided.

Asked whether he has any concerns about Trump’s ability to get Mexico to pay for a border wall, Simes shook his head.

“We think that with our economic might, [and] with sanctions, that we can influence China, that we can influence Russia. We even sometimes bulldoze the European Union,” Simes said, adding that “the idea that we’re totally impotent to influence Mexico — I find it very unpersuasive.”

Trump will speak at 12 p.m. ET on Wednesday at the Mayflower Hotel in downtown Washington, D.C.