President Joe Biden's emphatic Oval Office statement Wednesday that the US "fully, fully, fully" backs Sweden's bid to join NATO emphasized the extraordinary, legacy defining role he has played in defying 21st century Russian expansionism.

Yet as the war in Ukraine grinds deep into its second, bloody summer, its short-term trajectory -- and the shape of the conflict's ultimate resolution -- remain as uncertain as they were after the Russian invasion stalled early last year.

And several developments are underscoring the still high peril of the conflict as Biden prepares to head to Lithuania next week for a hugely symbolic NATO summit in one of the Baltic states once forcibly folded into the Soviet Union.

They include:

— A war of words between Ukraine and Russia over the Zaporizhzhia nuclear power plant after Ukrainian President Volodymyr Zelensky accused Russia of readying a bomb attack. The back-and-forth is raising fears of another alarming new dimension to a war already marred by alleged war crimes and terrible suffering among civilians.

— Western officials are still, meanwhile, assessing the impact of the suppressed Wagner group rebellion on President Vladimir Putin's political standing, and if it could lead him to more extreme steps in a war that has been a disaster for Russia.

— There is disappointment abroad that Ukraine's long-awaited offensive hasn't yet delivered a conflict-changing blow to Russian forces. Kyiv insisted on Wednesday that it is gaining momentum "gradually."

— Zelensky is escalating heat on the West for it to do more to help Ukraine, issuing an emotional call in an exclusive interview with CNN broadcast Wednesday for a full invitation to join NATO. The US says such a step is not likely with Ukraine involved in a full-scale war with Russia as it sticks to one of its own strategic aims -- avoiding a direct conflict with the nuclear superpower.

— The longer the war drags on without a decisive breakthrough on the battlefield, the more political pressure will grow on Ukraine's arms and funding lifeline -- especially in the US, where next year's presidential election is looming as a critically important factor.

Nuclear rhetoric

In the interview with CNN's Erin Burnett, Zelensky warned that he had intelligence that Russia was ready to carry out an attack on the Zaporizhzhia nuclear power plant. This followed a video address in which he said Moscow's forces had placed "objects resembling explosives" on roofs at the facility.

In response, Kremlin spokesman Dmitry Peskov warned that the potential for "sabotage" by Ukraine at the plant was high. Given Russia's previous nuclear rhetoric and fog of war misinformation, many Western observers are likely to give Zelensky the benefit of the doubt. It's plausible that any explosions at the plant set by Moscow's forces could subsequently be claimed by the Kremlin to be the result of Ukrainian shelling in a false flag scenario.

Zelensky's comments appeared to reflect the Ukrainian president's concern that his Western allies and UN nuclear regulators are not taking the situation sufficiently seriously. But the lack of specificity of his charges also shows how hard it is for outsiders -- at least those who lack access to intelligence -- to know exactly what is going on in Ukraine. The International Atomic Energy Agency said in an update on Wednesday that there were no visible indications of mines or explosives at the nuclear power plant but that it was essential to get better access to key areas of the vast facility.

Repeated alarms about the risks to the Zaporizhzhia nuclear power station have conjured nightmare scenarios for months. Still, CNN's Christian Edwards reported that the chances of an incident on the scale of the 1986 meltdown at Chernobyl had been limited since reactors had been placed into a safer cold shutdown mode. But an explosion could still release a radioactive plume into the air and seed a danger zone that could linger for decades.

The fact that there is even a conversation about the relative scale of a potential nuclear incident potentially caused by deliberate action underscores the depravity of the Ukraine war -- and the extent to which an evolved Russian war aim appears to be ravaging vast tracts of the country, even if it can't be conquered.

How will Putin's behavior be shaped by his political position?

Zelensky also used his interview with CNN to discuss the stunning political events in Russia late last month, which ended when Belarus purportedly stepped in to negotiate an end to a rebellion by Wagner Group mercenaries led by Yevgeny Prigozhin as they appeared to be marching on Moscow. Western governments are wrestling with the question of whether Putin's political power base is weakened and could cause him to rain even greater vengeance on Ukraine as a distraction. There may, however, also be a chance for Ukraine to take advantage of the chaos in Moscow as it presses forward with its battlefield offensive.

Zelensky argued in the interview that the uprising exposed splits inside Russia over the war and an erosion of Russians' positions. "We all see this process that shows half of the Russian population is in serious doubt. All those stories that he controls everything; these are feeble stories now," Zelensky said of Putin. He predicted that the Russian leader would make extensive efforts to "consolidate his society."

Senior US officials argued last month that Putin had been diminished by the rebellion. And the Republican chairman of the House Intelligence Committee, Rep. Mike Turner of Ohio, told CNN's Jake Tapper on Wednesday that the one thing the revolt had "obviously done is weaken Putin." He also said that the fact that Prigozhin's men had been able to march toward Moscow suggested likely support among some Russian troops and that Putin would now have to fear cracks in his own military.

Zelensky admits Russia slowed Ukraine's offensive

The prospect of Ukraine's summer offensive -- and the possibility that it could create significant breaches in Russian defenses and reclaim some of its seized territory -- had a powerful galvanizing political impact and helped push Western governments to increase their pipeline of arms to Ukraine over the winter.

So any perception that it is falling short of those high hopes and that Ukraine cannot ultimately win could also be a complicating factor as US and allied leaders nurture public support for their multi-billion dollar aid program. Zelensky admitted to Burnett that the counter-offensive had been "slowed down" because of widespread Russian mine laying and defensive lines that are three layers deep in some places.

He also appeared to suggest that he could not start the operation as early as he wanted, owing to delays in getting more high-tech arms and ammunition from Western donors. "I wanted our counter-offensive to start much earlier," he said, so that the Russians would have less time to prepare.

The Ukrainian military, however, said Wednesday that it was making progress and driving Russia out of previously captured positions near the village of Klishchiivka in the Bakhmut sector. And Ukrainian Deputy Defense Minister Hanna Maliar said Wednesday that Kyiv's forces were "gradually advancing" and that Russian troops were not making any progress in areas where they were attacking.

The US home front

Washington's multi-billion dollar support for Ukraine and willingness to gradually send more and more sophisticated weapons systems have played a critical role in sustained resistance to the Russian invasion. But there are increasing doubts over the willingness of the Republican-controlled House to maintain the same level of support, with some pro-Donald Trump members arguing that Zelensky should be forced to discuss peace terms with Moscow.

Given Democratic support for the war, there do seem to be sufficient votes to get a high level of funding through the House, though Washington is at risk of grinding to a halt in a government shutdown in the fall and winter amid pitched clashes over spending that could slow Ukraine aid.

Senate Minority Leader Mitch McConnell insisted Wednesday that continued US backing for Ukraine was vital, especially since it would send a message of resolve to another US adversary, Chinese President Xi Jinping. "There are some in both parties arguing that this is not significant for us, what's going on in Europe," McConnell said at Fort Knox. "That's not my view, it's not the majority view of Republicans in the Senate and Democrats as well," the Kentucky Republican said.

McConnell is a throwback to a more traditional, internationalist brand of Republicanism and rejects the "America First" approach of Trump. The ex-president has vowed to end the war within 24 hours if he wins a new White House term in 2024, a likely sign he would seek to do so on terms favorable to Putin, whom he constantly flattered during his administration.

Zelensky knows that Washington's backing -- from arms and equipment, to the diplomatic direction Biden brings to the West -- will help dictate Ukraine's fate. In an emotional moment in his interview with Burnett, he called on Biden to welcome his country into NATO.

And he warned: "Without the US's help, it will become a frozen conflict. With the US's help, we will reoccupy our territory."