@Amon-Sul Soviet-Mongolian Defense Pact: Due to their mutual border conflict with Japan in 1939, the Soviet Union and Mongolia have a special relationship. The Mongolian territories (Olgiy, Dzavhan, Tsagaan-Olom, Central Mongolia, Ulaanbaatar, and Buyant-Uhaa) will never become pro-Axis unless one or more of them is attacked by the Soviet Union. Also, if Japan attacks any Soviet-controlled territory that is adjacent to any Mongolian territory, all Mongolian territories that are still strict neutral or pro-Allies, or have joined the Allies as a result of a failed Japanese attack, are placed under the control of the Soviet Union at the end of Japan’s Conduct Combat phase. This is done in the same manner as though the Soviet Union had moved land units into a friendly neutral territory (see “Friendly Neutrals,” page 10). These territories have Soviet control markers placed on them, and their standing army units are placed on the board using Soviet pieces and are controlled by the Soviet Union player from then on. This occurs regardless of the state of relations between the Soviet Union and Japan at the time of the attack, with one exception: If the Soviet Union attacks Korea or any Japan-controlled territory bordering these Mongolian territories while Mongolia is still a strict neutral, Mongolia will remain neutral and not ally itself with the Soviet Union.
Think it is as usual, that they will become Soviet if Japan attacks any bordering territory only.