BERLIN — President Vladimir Putin of Russia has sought to convince President Donald Trump that his troops are marching toward inevitable victory in Ukraine, arguing that Kyiv should hand over the entire eastern Donbas region to avoid impending defeat. But the situation on the battlefield tells a different story. After making gains late last year, the Russian military has slowed to a crawl. In some parts of Ukraine, it has lost territory. At its average monthly rate of advance so far this year, it would take Russia more than three decades to seize full control of the Donbas, which the Kremlin has set as a condition for ending the war. The slowdown may be temporary, and it is due at least in part to seasonal factors. Russian troops tend to pick up speed over the summer, aided by better weather and foliage that provides more cover from drones. In recent days, Ukrainian officials have warned that Moscow’s troops are gearing up for new offensives and have intensified operations across the front.…