PHILADELPHIA — Russia's escalating violence on Ukrainian cities has united America and Congress behind Washington's frantic effort to bolster Ukraine's defenses and minimize the civilian casualties.

But as House Democrats gathered here this week for their annual retreat outside the Beltway, there was an underlying frustration that the United States and its western allies, while unified, also have their hands tied.

While Washington policymakers are racing to get ammunition and certain weapons into the hands of Ukrainian resistance forces — military and civilian alike — the administration has drawn a red line, ruling out the possibility of engaging Putin's forces directly, by air, ground or sea, unless Russia takes the further step of advancing on a country officially aligned with NATO. Ukraine is not.

The United States, joined by much of the world, has applied stringent economic sanctions on Moscow. But so far that's done nothing to dissuade Russian President Vladimir Putin from continuing his bloody assault.

The televised tragedy unfolding in Ukraine has exposed the limitations of U.S. military forces and the broader influence of NATO and its allies in the face of a volatile strongman willing to invade a sovereign country without concern for civilian casualties.

And that's frustrating lawmakers on Capitol Hill, who are struggling to come up with an effective strategy for countering the violence, short of engaging Russian forces directly.

“I wish I had an answer,” said one Democrat. “I don't.”

Democrats came to Philadelphia to talk about their domestic legislative agenda and campaign messaging strategy heading into November's midterm elections, when they're facing high hurdles in their effort to keep control of the House. But the arrival of war in Europe — the largest conventional conflict since the Second World War — hung like a storm cloud over the pep rally, dominating the front pages and cable news coverage even as President Biden sought to rally his party behind a message of economic optimism.

It was a cloud that Democratic leaders freely acknowledged.

“This is central to what we're talking about here,” Speaker Nancy Pelosi (D-Calif.) told reporters Friday morning. “People are dying.”

The urgency surrounding Russia's invasion has swelled in recent days, as Putin's targets appear increasingly to be civilian areas: apartment buildings, a maternity hospital, a kindergarten. At least some of those attacks are coming from the air, experts say, which has fueled the appetite from some lawmakers in both parties for Biden to help Ukraine establish a no-fly zone over the embattled country.

It's an idea the president has rejected out of hand, warning that such a strategy would necessarily involve direct engagement between Russian forces and those of the United States and its NATO counterparts. And Democratic leaders are siding squarely with their White House ally.

Another proposal would have Poland deliver fighter jets to Ukrainian pilots, with the United States filling the void in Poland's air forces. But that strategy hit a snag this week, when leaders in Warsaw insisted that the jets not be delivered directly from Poland, but go through U.S. military bases in Germany — an idea the Pentagon has rejected.

“Poland has said they want to deliver them to a U.S. air base, which implicates the United States,” House Majority Leader Steny Hoyer (D-Md.) told reporters Friday in Philadelphia. “I don't want to give Putin any excuse.”

Rep. Gregory Meeks (D-N.Y.), chairman of the House Foreign Affairs Committee, had just returned from a trip to the Polish border, where he met with military leaders from Poland, Ukraine and the Pentagon.

“I actually thought that we were close to an agreement when I left Poland on Monday,” he said. “But things have shifted.”

At the time, Meeks said, the plan was to have Ukrainian pilots “come and fly back from Poland and then we would backfill the needs of the Polish government.”

“The change — and I think this is where Gen. [Mark] Milley and Secretary [Lloyd] Austin have concerns — to have the planes go to Germany ... is a dangerous matter. Because that could be then [interpreted as] we're at war with Russia.”

Biden, speaking to House Democrats in Philadelphia on Friday, issued a pointed warning to his allies in Congress: Any direct engagement with Russian forces will start another world war.

“The idea that we’re going to send in offensive equipment and have planes and tanks and trains going in with American pilots and American crews, just understand — don’t kid yourself — no matter what you all say, that’s called World War III, OK?” he said.

Biden said he speaks with Ukrainian President Volodymyr Zelensky almost daily, including a conversation earlier in the day that lasted “almost an hour.” Democrats have been energized by the defiance of Zelensky and the outgunned Ukrainian forces in the face of a much more formidable foe. But the limitations on U.S. intervention are clearly weighing on them.

“This convoy is a sitting duck, and everyone wishes you could get a few planes to start taking it out,” said Rep. Seth Moulton (D-Mass.), an Iraq War veteran, referring to the Russian convoy north of Kyiv. “But that's not something that we can do, because that would start a war with Russia.”