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But the prospect of greater U.S.-Russian cooperation on Syria is already a topic of discussion. As National Security Adviser John Bolton told CBS last week: "There are possibilities for doing a larger negotiation on helping to get Iranian forces out of Syria and back into Iran, which would be a significant step forward." Russian Foreign Minister Sergei Lavrov, meanwhile, said a complete Iranian withdrawal from Syria was not realistic, Bloomberg wrote.

If this sounds familiar, it should. This kind of bargain was discussed during the Obama administration; former Secretary of State John Kerry spent his last year in office following Lavrov all over the world in an attempt to create a U.S.-Russian framework for resolving the Syrian civil war. He failed.

Read alsoAt Trump-Putin summit, Trump to enlist Russia's help to oust Iran from Syria – media

And when Trump came into office in 2017, two former administration aides tell me, he initially sought a U.S.-Russian deal to combat Islamic State. In his brief tenure as national security adviser, Michael Flynn pursued such a deal.

But a strategic deal, as opposed to smaller regional ceasefires in Syria, was dropped following former Secretary of State Rex Tillerson's visit to Moscow in April 2017. It was a testy visit that soured Tillerson and other top officials on the chances for a broader agreement. Now that Tillerson is gone (along with Flynn's replacement, H.R. McMaster), Trump is liberated and wants to get to know Putin better – and gauge his willingness to help isolate Iran.

This is a pointless and dangerous gambit. First, by announcing his intention to pull U.S. forces out of the country "very soon," Trump has already given away much of his leverage within Syria.

Ideally, Trump would want to establish a phased plan with Putin, where the U.S. would make some withdrawals following Iranian withdrawals from Syria. But Trump has already made it clear that prior U.S. objectives for Syria, such as the removal of dictator Bashar al-Assad, are no longer U.S. objectives. The U.S. has also declined to make commitments to give money for Syrian reconstruction.

Without any leverage, Trump will have to rely even more on Putin's word, which is worthless. Putin to this day denies any Russian government role in interfering in the 2016 U.S. election. Just last month, Putin went on Austrian television and lied about his government's role in shooting down a Malaysian airliner over Ukraine. Why would anyone trust Putin to keep his word to help remove Iran and its proxies from Syria?

Read alsoAtlantic Council: Will Trump give away Crimea at Helsinki?

And this gets to the most dangerous possible outcome of the upcoming summit. The one thing that Kerry never did was attempt to trade concessions on Syria for concessions on Crimea, the Ukrainian territory that Russia invaded and annexed in 2014. There was a good reason for this: Even if one argues that the future of Ukraine is not a high priority for U.S., it's a disastrous precedent to allow one nation to change the boundaries of another through force, particularly one that signed an agreement with the U.S., U.K. and Russia to preserve its territorial integrity in exchange for relinquishing its cold-war era nuclear weapons.

Yet Bolton and other U.S. officials will not say whether Trump will continue his current policy of imposing sanctions on Russia for its annexation of Crimea and interference in Ukraine. Their reticence suggests Trump is at least willing to discuss cooperation on Syria in exchange for abandoning Ukraine.

Borys Potapenko, a Ukrainian-American activist and an occasional intermediary between Kyiv and Washington, met last week with U.S. officials and has adjusted his expectations accordingly. "The best we can hope for is that the two leaders are content with establishing a better personal relationship," he said. Officials assured him that there was no change to U.S. policy on Ukraine, he said, but also told him that the final decision rests with the president.

That's what worries him – and should worry the rest of us as well, Bloomberg says.