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….