The USSR kept its contributions to the Korean War secret as much as they could. The North Korean air force was originally largely comprised of Soviet planes and Soviet pilots, for example. And much of the materiel (such as guns, bullets, artillery, tanks, etc.) used to prosecute the war was donated by the USSR. But the Soviets never dedicated ground troops, largely because they wanted to maintain plausible deniability in public for their contribution to the war and also to avoid a direct confrontation between the US and the Soviet Union, which likely would have escalated to nuclear warfare.