South Korean defense firms have long been active in Europe, selling mobile howitzers and small arms to a number of countries over the past half-decade. But those deals have paled in comparison to the $5.8 billion blockbuster contract Poland signed with two South Korean companies in July. The agreement included 980 K2 Black Panther tanks, 672 K9 self-propelled howitzers and 48 FA-50 fighter planes, with deliveries on some of the tanks and howitzers expected to begin by the end of this year. The deals could be worth as much as $15 billion if all of the options are exercised in...