The Winter War
After the Molotov-Ribbentrop pact in August and the occupation of the Baltic countries in September, the Soviet Union turned its eyes towards its northernmost neighbour, Finland. An ultimatum, demanding the surrender of Viiburi to the Soviet Union, was issued, but rejected, followed by Soviet declaration of war on the 15th of November.

Soviet order of battle:
In parentheses approximate equipment is marked for armor divisions
STAVKA: Boris Shaposhnikov
Chief of the Army: Klimenty Voroshilov
Land Forces
1st Mechanized Army - Marshal of the Soviet Union Ivan Konev - Based in the Leningrad area
1st Mechanized Corps - General Vassilevskij3rd Tand Division (T-28s and BA-20s) - General Vassilevskij
5th Tank Division (T-28s and BA-20s) - Lt. General Romanenko P.L.
82nd Motorized Division - Lt. General Homenko
2nd Mechanized Corps - Marshal Konev7th Tank Division (T-28s and BA-20s) - Marshal Konev
8th Tank Division (T-28s and BA-20s) - Lt. General Volskiy
213th Motorized Division - Lt. General Tamruchi
3rd Motorized Corps - Lt. General Novikov V.V.9th Tank Division (T-28s and BA-20s) - Lt. General Novikov V. V.
10th Tank Division (T-28s and BA-20s) - Lt. General Solyankin
205th Motorized Division - Lt. General Sofronov
4th Mechanized Corps - General Pavlov D.G.12th Tank Division (T-28s and BA-20s) - General Pavlov D.G.
11th Tank Division (T-28s and BA-20s) - Lt. General Trubnikov
29th Motorized Division - Lt. General Gordov
5th Mechanized Corps - Lt. General Terekhin1st Tank Division (BT-5s and BA-20s) - Lt. General Terekhin
2nd Tank Division (BT-5s and BA-20s) - Lt. General Golikov
84th Mechanized Division* - Lt. General Eremenko
Leningrad Front - Marshal of the Soviet Union Semyon Timoshenko - based in the Ladoga area
Positioned on the southwestern bank of Ladoga:
Leningrad Front HQ - Marshal Timoshenko
2nd Corps - Lt. General Krasnopevtsev142nd Rifle Division - Lt. General Kransopevtsev
161st Rifle Division - Lt. General Gastilovich
235th Rifle Division - Lt. General Shumilov
3rd Corps - Lt. General Voronov48th Rifle Division - Lt. General Voronov
43st Rifle Division - Lt. General Poluektov
10th Rifle Division - Lt. General Zakharvo M.V.
4th Corps - Lt. General Fedorenko177th Rifle Division - Lt. General Fedorenko
191st Rifle Division - Lt. General Reiter
8th Rifle Division - Lt. General Tyulen
Positioned on the northeastern bank of Ladoga:
5th Corps - General Levandovski32nd Rifle Division - General Levandovski
40th Rifle Division - Lt. General Trofimenko
189th Rifle Division - Lt. General Sandalov
6th Corps - Lt. General Berzarin12th Rifle Division - Lt. General Berzarin
3rd Rifle Division - Lt. General Kurochkin P.A.
92nd Rifle Division - Lt. General Lutchinski
7th Corps - Lt. General Malinin190th Rifle Division - Lt. General Malinin
197th Rifle Division - Lt. General Kuznetsov V.V.
199th Rifle Division - Lt. General Maladin
Karelian Front - General Frolov - Positioned between White Karelia and Murmansk
Located near Kem:
1st Corps - Lt. General Malinovskij136th Rifle Division - Lt. General Malinovskij
25th Rifle Division - Mj. General Chibisov
176th Rifle Division - Lt. General Tolbukhin
Located between Kem and Kandalaksja:
Karelian Front HQ - General Frolov
2nd Mountaineer Corps - Lt. General Kholostyakov138th Rifle Division** - Lt. General Kholostyakov
47th Rifle Division** - Mj. General Kutlin
17th Rifle Division** - Mj. General Pisarevskij
3rd Mountaineer Corps - Lt. General Kuznetsov A.I.63rd Rifle Division** - Lt. General Kuznetsov A.I.
60th Rifle Division** - Lt. General Semenovskij
95th Rifle Division** - Lt. General Marchenkov
Located near Murmansk:
1st Mountaineer Corps - Lt. General Batov68th, 83rd, 194th Rifle Divisions*** - Lt. General Batov
21st Rifle Division** - Lt. General Rakutin
101st Rifle Division** - Lt. General Ryabchev M.E.
Total land forces available for the invasion of Finland:
30 Infantry Divisions (Including 9 mountaineer divisions)
10 Tank Divisions (with BA-20 brigades)
5 Motorized Divisions
About half a million men.
Air forces
Chief of the Air Force: Pavel Rychagov
Based in Leningrad
1st Bomber Fleet**** - Air General Yakovlev
4 bomber wings (Ilyushin DB-3), attachment escort fighters (Yakovlev Yak-2)
1st Fighter Fleet**** - Mj. General Rychagov
4 fighter wings (Lavotshkin LaGG-3)
Based in Murmansk
1st CAS Fleet**** - Air General Novikov
4 wings of CAS bombers (Sukhoi Su-2), attachment escort fighters (Yakovlev Yak-2)
Total of 12 air wings
Navy
Chief of the Navy: Mikhail Viktorov
Baltic Sea Fleet - Grand Admiral Kuznetzov
Marat - Gangut-class Battleship
Oktyabrskaya Revolucija - Gangut-class Battleship
1st Destroyer Flotilla - Norvik-class destroyers
4th Destroyer Flotilla - Gnevnyi-class destroyers
1st, 2nd, 3rd, 4th, 5th, 6th, 7th Submarine Flotillas - Series II-class submarines
Total of 2 capital ships, 2 screening flotillas, 7 submarine flotillas

*Despite being named a mechanized division, it was still a strictly motorized force, being created out of the 188th Rifle Division amongst the first motorized formations.
**A mountaineer division named as a regular rifle division.
***These "three rifle divisions" actually form one mountaineer division
****An Air Fleet is the equivalent of an American Air Wing.