Their cover blown, Blinken and Austin pushed on to Kyiv despite the security risks.

IN POLAND, NEAR THE UKRAINIAN BORDER — Nearly 48 hours since leaving Washington on what was supposed to be a clandestine mission, Secretary of State Antony J. Blinken and Defense Secretary Lloyd J. Austin III met with reporters to discuss an operation that was nearly over before it began.

“This was an important time to be there,” Mr. Blinken said Monday morning. “An important time for Ukraine, for the war, and an important moment to have face-to-face conversations in detail.”

The visit also resulted in a striking redefinition of success for America’s goals in Ukraine.

“We want to see Russia weakened to the degree that it can’t do the kinds of things that it has done in invading Ukraine,” Mr. Austin said. “It had already lost a lot of military capability and a lot of its troops, quite frankly, and we want to see them not have the capability to very quickly reproduce that capability.”

The two cabinet secretaries spoke in a warehouse standing in front of tall stacks of humanitarian aid, while across from them were green-painted wooden boxes of munitions for Soviet-designed weapons used by Ukrainian troops — striking visual reminders of the kinds of aid the United States is providing Kyiv.

All of it would be inside Ukraine’s borders by the end of the day, Mr. Blinken said.

Last week, Mr. Blinken’s and Mr. Austin’s staffs were planning a trip to an Air Force base in Ramstein, Germany, to meet with officials from other nations on Tuesday to discuss ways they could help Ukraine in its fight against Russia. A handful of those same staffers worked in parallel, on a need-to-know basis, to plan a stop beforehand in Kyiv so the secretaries could personally inform President Volodymyr Zelensky of Ukraine that President Biden would quickly re-establish the United States’ embassy in the Ukrainian capital and provide hundreds of millions of dollars in additional military aid. It would be an unannounced trip by the highest-level delegation of American officials since the Russian invasion began.

Participants who ended their week thinking they would be leaving for Germany on Monday morning were told on Friday afternoon that plans had changed.

Both Mr. Blinken and Mr. Austin boarded military C-17 transport planes early Saturday morning at Joint Base Andrews in Maryland, driving directly onto the tarmac and lifting off with the secrecy of their mission intact.

That secret held until about halfway through the nine-hour flight to Poland.

In an apparently unscripted remark Saturday afternoon during a news conference in the Ukrainian capital, Mr. Zelensky announced that Mr. Blinken and Mr. Austin would be arriving in Kyiv the next day. Shortly afterward, a senior defense official emerged from Mr. Austin’s quarters in the C-17’s cargo hold and somewhat sheepishly informed the three reporters accompanying the defense secretary that President Zelensky had blown the operation’s cover and the future of the trip was uncertain.

However, Pentagon officials had planned for a number of contingencies, and having details of the secret trip leak out was among them. So the two cabinet members’ planes pressed on.

After arriving in Poland early Sunday morning, Mr. Blinken and Mr. Austin climbed into cars and were driven across the border into Ukraine to begin an 11-hour train ride to Kyiv. They were accompanied by just a few of their staff members, and their location was tracked minute by minute in a U.S. military tactical operations center in Poland.

While the secretaries were en route to Kyiv, a senior State Department official and a senior defense official offered reporters in Poland a preview of what Mr. Blinken and Mr. Austin would be offering to Mr. Zelensky.

The officials did not have any information on whether Marines would be posted to guard the embassy in Kyiv once it reopens, but they did add that Mr. Biden plans to quickly nominate an ambassador to lead it.

According to the senior defense official, the first group of more than 50 Ukrainian artillery soldiers on Sunday completed their training on 155-millimeter howitzers provided by the United States, which are slightly different in design to the Soviet-era 152-millimeter guns that have been used by Ukrainian forces since the country gained independence. A second group of Ukrainian artillery specialists would soon begin another six-day training course, the official added.

The cabinet secretaries returned to Poland, near the Ukrainian border, Monday morningafter traveling nearly nonstop over the previous two days.

Immediately afterbriefing reporters, Mr. Blinken and Mr. Austin met with soldiers from the 82nd Airborne Division and thanked the troops for their service, then boarded Air Force cargo planes to head to Ramstein.

In Germany, Mr. Blinken and Mr. Austin will be joined by the NATO secretary general, Jens Stoltenberg, and the Ukrainian defense minister, Oleksiy Reznikov, to offer a battlefield update to officials from more than 20 nations. They are also expected to discuss the deployment of new military aid to Ukraine and how each country can use their own defense industries to produce goods in high demand by Ukraine.

“The first step in winning is believing that you can win,” Mr. Austin told reporters. “And so they believe that we can win, we believe that they can win if they have the right equipment, the right support.”

“We’re going to do everything we can — continue to do everything we can to ensure that gets to them,” he added.