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, such as 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 Zelenskyy 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.