Ukraine and Russia poured reinforcements into eastern Ukraine this weekend, preparing for what is likely to become the war’s biggest battles as refugees continued to flee the looming Russian assault. Russia’s main objective now is to seize the parts of the eastern Donbas region not yet controlled by Moscow. Unlike the first phase of the six-week-old conflict, that shift is forcing Ukraine into fighting conventional battles involving tanks, artillery and aircraft on flat, often barren terrain that allows Russia to leverage its superiority in military equipment.