More than two years into their wartime alliance, the bond between the United States and Ukraine is showing signs of wear and tear, giving way to mutual frustration and a feeling that the relationship might be stuck in a bit of a rut.

It is the stuff that often strains relationships — finances, different priorities and complaints about not being heard.

For the Pentagon, the exasperation comes down to a single, recurring issue: American military strategists, including Defense Secretary Lloyd J. Austin III, believe that Ukraine needs to concentrate its forces on one big fight at a time. Instead, President Volodymyr Zelensky, who has vowed to drive Russia out of every inch of Ukraine, expends his forces in battles for towns that U.S. officials say lack strategic value.

The most recent example involved the battle for the eastern city of Avdiivka, which fell to Russia last month. U.S. officials say Ukraine defended Avdiivka too long and at too great a cost.

For its part, Ukraine is increasingly disheartened that American political paralysis has resulted in shortages of ammunition for troops on the front. As each day goes by without a fresh supply of munitions and artillery, and Ukrainian crews ration the shells they have, morale is suffering.

Mr. Zelensky promised a “renewal” of Ukraine’s military in its stagnant campaign against Russia when he dismissed his commanding general, Valery Zaluzhny, last month and named Gen. Oleksandr Syrsky, the head of his ground forces, to replace him.