Through fields that before the war supplied Ukrainian grain to world food markets, we now stepped gingerly between antitank mines laid by Russian President Vladimir Putin’s “liberators.” Around us, a dozen young women and men wearing soft body armor poked at the earth with bayonets. Rich farmland that once fed the world — 174,000 square kilometers, an area the size of Florida — now offered only the promise of decades of dangerous work to clear the millions of mines the Russian military has sown since Russia’s full-scale invasion two years ago. As veterans in Ukraine to assist with humanitarian aid...