Trump promised to end the war in Ukraine. Now he must decide how.

Throughout his campaign for the White House, Trump bashed Biden’s handling of Ukraine, warning that it made World War III more likely and that Kyiv fleeced the U.S. by obtaining weapons worth billions of dollars free of charge. He has said he could resolve the conflict quickly, bringing both sides to the negotiating table, but hasn’t revealed how he would do so.

“I can’t give you those plans because if I give you those plans, I’m not going to be able to use them," he said during the campaign.

In fact, Trump hasn’t approved a specific peace plan, allies said, including how he would persuade Russian President Vladimir Putin and Ukrainian President Volodymyr Zelensky to sit at the same table and negotiate. While advisers have offered ideas, only the president-elect, in consultation with key aides, will decide how to handle the sensitive and high-stakes diplomacy, they said.

“Anyone—no matter how senior in Trump’s circle—who claims to have a different view or more detailed window into his plans on Ukraine simply doesn’t know what he or she is talking about or doesn’t understand that he makes his own calls on national-security issues, many times in the moment, particularly on an issue as central as this," a former Trump National Security Council aide said.

Like in Trump’s first term, different factions are set to compete to influence the Republican’s foreign policy. More traditionally minded allies such as Mike Pompeo, the former secretary of state now in contention to lead the Pentagon, are likely to push for a settlement that doesn’t appear to give a major win to Moscow. Other advisers, particularly Ric Grenell, a top candidate to lead the State Department or serve as national-security adviser, could give priority to Trump’s desire to end the war as soon as possible, even if it means forcing Kyiv into significant concessions.

The proposals all break from Biden’s approach of letting Kyiv dictate when peace talks should begin. Instead, they uniformly recommend freezing the war in place—cementing Russia’s seizure of roughly 20% of Ukraine—and forcing Ukraine to temporarily suspend its quest to join the North Atlantic Treaty Organization.

One idea proposed inside Trump’s transition office, detailed by three people close to the president-elect and not previously reported, would involve Kyiv promising not to join NATO for at least 20 years. In exchange, the U.S. would continue to pump Ukraine full of weapons to deter a future Russian attack.

Under that plan, the front line would essentially lock in place and both sides would agree to an 800-mile demilitarized zone. Who would police that territory remains unclear, but one adviser said the peacekeeping force wouldn’t involve American troops, nor come from a U.S.-funded international body, such as the United Nations.

“We can do training and other support but the barrel of the gun is going to be European," a member of Trump’s team said. “We are not sending American men and women to uphold peace in Ukraine. And we are not paying for it. Get the Poles, Germans, British and French to do it."

That proposal in some respects echoes comments made by Vice President-elect JD Vance during a September interview, when he suggested a final agreement between Ukraine and Russia could involve a demilitarized zone “heavily fortified so the Russians don’t invade again." Russia, Vance continued, would get to keep the land it has taken and be assured of Ukraine’s neutrality.

“It doesn’t join NATO, it doesn’t join some of these sort of allied institutions," he said on “The Shawn Ryan Show," a podcast.

Earlier this year, Keith Kellogg and Fred Fleitz, who both served in Trump’s first White House, presented Trump with a blueprint that includes withholding weapons from Ukraine until Kyiv agrees to peace talks with Russia. Ukraine could still try to regain lost territory, but would have to do so through diplomatic negotiations.

Which of these strategies Trump will pursue—in whole, in part, or at all—is unclear. But any gambit to start Ukraine-Russia peace talks, let alone complete an agreement, faces many pitfalls.

For one, Ukraine and Russia still have vastly differing war aims and little desire to alter them. With Russian troops advancing slowly but steadily in Ukraine, the Kremlin has shown little inclination to negotiate, and has shown its willingness to escalate the conflict with hybrid attacks outside its borders, such as sabotage operations in Europe.

“The objectives of the special military operation remain unchanged and will be achieved," Dmitry Medvedev, a top Russian official, posted Wednesday to X after learning of Trump’s victory over Vice President Kamala Harris, the Democratic presidential nominee.

There will also be stiff resistance from some NATO allies, who view Russia’s aggression against Ukraine as an immediate threat to their own countries. Elina Valtonen, Finland’s foreign minister, said she wasn’t aware of any specific proposals by Trump’s team but stated “there shouldn’t be any negotiation without Ukraine agreeing to the fact that it’s negotiating, and doing so on its terms."

Zelensky, whose country is heavily dependent on the U.S. for military and financial assistance, could—more easily than Putin—be forced by Trump to negotiate, but the Ukrainian leader would have to contend with a public that views ceding territory as capitulation to Moscow.

Trump has said that Ukraine’s survival is important to the U.S., but has repeatedly criticized Zelensky, calling him the “greatest salesman," a stance that has worried some officials in Kyiv that a Trump-led U.S. might push for a settlement that favors Russia.

Zelensky on Wednesday congratulated the president-elect on his victory, appealing to their September meeting in New York and praising his “ ‘peace through strength’ approach in global affairs."

The Ukrainian leader also said Wednesday that he had spoken on the phone with Trump, adding that both men “agreed to maintain close dialogue and advance our cooperation."

Alan Cullison contributed to this article.

Write to Alexander Ward at alex.ward@wsj.com