WASHINGTON — For months, Pentagon officials have warned that without action from Congress, the Defense Department would quickly burn through the remaining security aid for Ukraine. Now they’re saying the tank is almost empty.

“It’s fumes,” said Undersecretary of Defense for Acquisition and Sustainment Bill LaPlante, speaking with reporters at the Reagan National Defense Forum over the weekend.

The dwindling aid presents a messaging challenge for White House and Pentagon officials. On one hand, they don’t want to portray Ukraine’s self-defense as fragile, liable to sputter without American support. On the other, they want to communicate a sense of urgency that could help free a $105 billion supplemental funding bill, of which more than half would go to Ukraine, from the slough of congressional negotiations.

Last night, the Senate voted down a version of that supplemental 49-51, along party lines. A small group of senators is negotiating a deal on U.S. border policy, which Republicans see as a political pay-for.

Meanwhile, of the two buckets of America’s military support for Ukraine, one is already empty and the second is dwindling. The administration has $4.8 billion left in authority to donate weapons to Ukraine, but only $1 billion left to replenish those stocks.

Doug Bush, the Army’s acquisition chief, said during a Defense One conference in Washington Thursday the service will soon send Congress a request to spend the remaining $1 billion. Congress has 15 days to approve or deny the request; none have been denied so far.

Assuming Congress approves the request, Bush said, “that would put us at a couple of weeks [until] that account is literally zero.”

Without further U.S. aid, Kyiv’s operations next year will be limited somewhat, a senior American military official, speaking on background to discuss sensitive topics, told Defense News. Last year at this time, some of the kit Ukraine needed for its counteroffensive was already arriving.

“We don’t want to wait until the spring,” the official said.

The counteroffensive is nearing its close, after Ukraine’s armed forces failed to take back anywhere near the territory it hoped for earlier in the summer. The lack of operational success combined with a slower trickle of aid may force Kyiv to reconsider its plans for the coming year — perhaps needing to concentrate its efforts on the 600-mile front line or revise its strategy to match fewer resources.

“What’s your plan B?” the official asked.

European countries such as the Netherlands and Germany have recently announced new commitments to support Ukraine, but American assistance has been about half of all the $100 billion in aid sent by Western countries thus far.

This week, representatives from Ukraine’s government and defense industry traveled to Washington for a three-day conference with American and European officials on building Kyiv’s wartime industrial base.

“It’s not as if there’s going to be an immediate drying up of support,” said a senior NATO official, speaking with reporters on the condition of anonymity in Washington.

As to the effect of the effect of stalled U.S. aid on the battlefield, the NATO official said it’s too early to tell.

Still, the Pentagon and administration continue to say U.S. aid is critical to keeping together the coalition of countries supporting Kyiv — and that they’re expecting the supplemental to pass.

“It’s going to put Ukraine in an increasingly vulnerable position just to hold the ground they have,” said Adam Smith (D-Wash), the ranking member of the House Armed Services Committee, in an interview with Defense News. “We’re past time on this.”

Noah Robertson is the Pentagon reporter at Defense News. He previously covered national security for the Christian Science Monitor. He holds a bachelor’s degree in English and government from the College of William & Mary in his hometown of Williamsburg, Virginia.

Jen Judson is an award-winning journalist covering land warfare for Defense News. She has also worked for Politico and Inside Defense. She holds a Master of Science degree in journalism from Boston University and a Bachelor of Arts degree from Kenyon College.

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