For years, North Korea’s military has helped its leader, Kim Jong-un, keep control of his people and provide a buffer against the country’s sworn enemy, South Korea. With 1.3 million members, the North’s army is among the world’s largest conventional armed forces.

Now, with more than 11,000 North Korean troops fighting alongside Russian forces against Ukraine, it’s playing a more prominent role in Mr. Kim’s geopolitical gambit for much-needed cash and diplomatic leverage.

The troops that North Korea deployed are from its “​Storm” Corps​, special forces that are among the military’s best trained ​and most heavily indoctrinated. But they were badly prepared for​ drone attacks and the unfamiliar terrain far from their isolated homeland, according to South Korea’s National Intelligence Service.

More than 100 of them were killed and 1,000 others wounded in their first battles, the intelligence agency told South Korean lawmakers in a briefing on Thursday. The agency said a general-ranking officer may be among those killed, according to Lee Seong-kweun, a lawmaker who spoke to reporters after the closed-door briefing.

The agency said Mr. Kim appeared to be preparing to send more troops to Russia, as he sees Europe’s biggest conflict since World War II as an opportunity to advance his own military and diplomatic ambitions.

Here is what to know about the North Korean military and the troops Mr. Kim sent in his country’s first major intervention in an overseas conflict.