On November 14, 1939 Finland refused to turn over the disputed border region to the USSR, and the USSR promptly invaded, starting the Winter War. The forces involved were:
Finland:
2 fighters, 15 Infantry Divisions (2 corps at Viipuri, 1 at Sortavala, others scattered)
Fleet: 2CA 2DD 2Sub
USSR: Total of 11 aircraft, 27 land divisions
Lenningrad: 1 Infantry Corps, 7 fighters, 4 tactical bombers
Murmansk: 2 Infantry Divisions
Petrozavodsk: 1 Shock army, 1 infantry corps, 1 motorized corps, 2 armored corps, 1 tank corps (understrength)
Fleet: 3BB 2DD 5Sub
Now, this is clearly not an equal fight (especially since the Red Army could come up with another 30 divisions or so without much risk), but Finland can put up quite a fight. I'm going to use the Winter War to detail the techniques I use while attacking, this should give you an idea of what's in the details of later battles (I'm not writing them up in as much detail).
The first step in fighting a war is to decide what your objectives are. In this case, it's easy - my goal is to completely annex Finland, thus securing Lenningrad against assault from the north. This is clearly an achievable goal, so I look next at what I have to accomplish to reach it. Annexation requires the occupation of all VP provinces in a country, so I need to hold the southern loop of Finland's coast (Viipuri, Helsinki, Turku, Pori, Vaasa). If you're not sure where a country's VP provinces are, go to the ledger and sort page 1 by 'controller' to get a complete list, though you'll still have to click around to locate them exactly. Now, it would be possible to take those provinces by simply smashing into Viipuri and driving up the coast, but this would be costly because Viipuri is fortified. My plan is to send armored forces behind Viipuri to encircle it and launch a coordinated attack, which will make the fight much, much easier and less costly. Some armor will try to speed along the coast to sieze the other key provinces, and if it is blocked then the bulk of the forces used against Viipuri can easily assist in clearing out the Finns. Infantry in Murmansk and Lenningrad (my only nearby key provinces) will prevent the Finns from siezing one of them to prolong the war.
So, on November 14th everything but one infantry corps sets out on a coordinated attack to assault Sortavala at dawn on Dec 4 (ctr-click to pick a date, then pick the earliest daylight time) supported by the Red Air Force from Lenningrad. The long delay is due to the low infrastructure rating at Sortavala. On December 4th, the Finns manage to hold out for only three hours before falling back. All of the units from this attack except for the motorized corps drive on to Mikkei. Since there are no units at Mikkei, I don't tell them to arrive at a particular time.
In other news, on December 8th Germany decides not to establish Vichy France after capturing Paris but to go for total conquest. Later on, I elect not to purge a minister who's name I don't recognize. I really wish that event would tell you what he is minister of.
On December 23rd, Zhukov's tanks rout the Finns who had retreated to Mikkei and the armored and tank units all drive on to Helsinki, leaving the 5th Shock Army to hold Mikkei (once they arrive). Notice that while the tanks are rushing on ahead, there is a chain of units along their supply line. When a major force is attacking, it is generally necessary to cover their entire supply line with troops so that your opponent can't just walt a single unit into it. You don't want to end up with your strongest units stuck in place and weakening from lack of supply while enemy units rush by like your attack didn't even happen. This is even more important now, as encircled enemy units will launch desperation attacks to get free from encirclement.
As Tukachevsky's tank corps takes control of Helsinki on December 27th, the Finns offer their border provinces for peace. Since there doesn't appear to be any real obstacle to annexation, I refuse that offer and prepare to wipe them out. On January 2nd, as Tukachevsky races along the Finnish coast, all other forces launch an attack on the Viipuri fortifications from 4 directions and supported by air power on the 9th. The units holding open the supply line now serve to hit the defending units from as many directions as possible:
On January 12th at 1800 hours, Finnish troops retake Helsinki while most of our army is tied up in the battle for Viipuri. This cuts off Tukachevsky, so the Red Banner Baltic Fleet sails out to guard a supply convoy. Two hours after the Finns liberated Helsinki, Zhukov and Konev win the battle of Viipuri and both armored corps move out to counter-liberate it. The Viipuri fight was brutal, the units involved all lost about 1/6 of their strength. This is a lot less than they would have lost in a frontal assault, however, and isn't that big of a deal to the USSR as a whole.
On the 14th, Zhukov liberates Helsinki and drives on Tampere to catch the retreating units while Konev sweeps up the coast to help Tuckachevsky, whose army has stalled. Motorized forces leave Viipuri to secure Helsinki just to be sure. By the 18th, Tukachevsky retreats from Pori at less than 25% strength, so on the 19th Zhukov secures Tampere, then drives on Pori to be sure that Konev does not have the same problem. The defenders at Pori are crushed after a 27 hour battle starting on January 21, and Tukachevsky recieves reinforcements before being redeployed out of Finland. On January 28s at 1000 hours, Konev finally reaches Vaasa and disrupts the unprepared forces there, bringing the Finnish campaign to a successful close.