He said the operation must be viewed as part of a larger whole.

“For me, every success during this war becomes a new stage, a new step, on the road to victory,” Mr. Reznikov said. The counteroffensive, he said, will be “just one story” in the war.

Military analysts have pointed to a likely period of probing assaults, feints and long-range strikes in the opening phase of the attack. Degrading the Russian military’s combat abilities will be as important as liberating territory, Mr. Reznikov said.

The Ukrainians see their enemy as having expended its offensive ability and as eager for a pause in fighting that could buy time to rearm and attack again.

Despite Ukraine’s worries about waning Western support, its allies have so far remained resolute, pledging hundreds of billions of dollars in weapons and aid, training Ukrainian soldiers, imposing sanctions and, to varying degrees, weaning their economies off Russian energy. NATO’s secretary general, Jens Stoltenberg, has said the alliance must brace itself to back Ukraine over a long war, and has singled out a summit planned for July in Lithuania as a moment to formalize that commitment.

In Washington, President Biden has pledged to support Kyiv for “as long as it takes,” and could request an additional supplemental aid package for Ukraine later this year, regardless of the counteroffensive’s outcome. Administration officials expect to retain bipartisan congressional support.