Soldiers fight in freezing, muddy trenches bombarded by artillery, or in warrens of burned and blown-up houses in urban combat. Casualty rates are high, and dangerous missions, like storming enemy-held tree lines, abound.

As they planned for a renewal of Ukraine’s military under extreme conditions, both the country’s former top commander and his replacement have emphasized the same looming problem: a need to relieve exhausted, battered troops whose combat tours have stretched nearly two years.

In a tumultuous week for Ukraine’s war effort, President Volodymyr Zelensky removed his commanding general, Gen. Valery Zaluzhny, on Thursday, while aid from the country’s largest source of weapons and ammunition, the United States, hung in doubt in Congress.

While Ukraine relies on allies for weaponry, replenishing the ranks is a domestic challenge. Small protests have broken out in opposition to a Parliament proposal to expand the draft to include younger men, but so far, lawmakers have slow-walked the measure.

Military analysts have mostly coalesced around the idea that Ukraine will, at best, hold existing front lines in ground fighting this year with a new influx of American weaponry — and risk falling back without it. It plans to replenish its ranks through mobilization while keeping Russia off balance with long-range drone strikes and sabotage operations behind enemy lines and inside Russia.

In announcing the appointment of Gen. Oleksandr Syrsky to command the military, Mr. Zelensky said he wanted a “new management team” for the armed forces. He has signaled a search for a new strategy that accounts for exhausted frontline soldiers in Ukraine’s million-man army, which is fighting the largest war in Europe since World War II.