In return for its arms and ammunition sent to Russia, North Korea might have received as much as $5.5 billion, an analyst told WSJ.By deepening military cooperation with Moscow, Pyongyang hopes to gain practical battlefield experience and the ability to modernize its weapons systems, said.North Korea has emerged as Russia's leading ally in the full-scale war against Ukraine, providing not only ballistic missiles and artillery ammunition but also soldiers. The two nations signed a defense treaty in June, requiring either state to render military aid to the other in the event of an attack.After reporting the first clashes with North Korean troops in the fall, Ukraine in December announced that Russia had begun using North Korean units in ground assaults in Russia’s Kursk Oblast.South Korean officials have reported that at least 100 North Korean soldiers have been killed while fighting for Russia against Ukraine.