There's not many routes to invade Southern Finland that aren't blocked by lakes, wetlands and lots of forests. And as per OTL the Russians sent their Arctic Troops, Naval Infantry and most famously their Airborne/VDV forces into Ukraine, all of which suffered considerable losses in the conflict. It might be less here but there'd still be a drain on Russian forces both from that Special Military Operation and the ongoing occupation of Ukraine. Plus the depletion of their guided missiles. Not as much as OTL but still hundreds if not thousands of missiles expended.
And there's like only a handful of road border crossings in the entire length of the Finnish-Russian border in the central and northern part of the border. And almost all of that border is heavily forested as well.
Plus Finland already has lots of trained reservists in addition to the active military and supposedly room to train up 900,000 troops total in six to nine months time. With NATO (and Scandinavian) support directly that could be performed in Norway and Sweden and other safe areas closeby.
When Russia attacks, they'll have to deal with a lot of anti-tank and anti-armor weapons. According to Wikipedia its 3000 NLAW's, almost 40,000 112mm Apilas, 70,000 M72 LAWS, 500+ TOW ATGM's and 200 SPIKE ATGM's. Yikes! The Russians lost lots of logistical vehicles and armored units in their opening moves tramping through the forest areas of Northern Ukraine and in the suburbs of Kyiv where reportedly Ukrainian forces armed with anti-armor weapons incurred impressive amounts of casualties upon the Russians in men and material.
And unlike Ukraine with those big ol fields in the South and East of the country, a lot of Finland is again wetlands and forest so it'd be bad everywhere and help negate Russia's potential advantages in artillery as well as in things like air support or drone support/spotting due to all of the foliage and concealment etc.
Russia still would have a big advantage in guided missiles (until they run out) and lots of artillery. Finland does have 29 M270 MLRS's though and Russia has been incapable of striking Ukrainian HIMARS so far, I can only imagine how much more of a headache supplying Russian advances with their depots would be when they invade Finland upon predictable axis and with no prior experience of enduring HIMARS strikes before. Plus Finnish MRLS' have access to extended range GMLRS and would far more likely get ATACMS as well.
That leads into the other thing. Finland would be easier to supply then Ukraine. They already use M270's so giving Finland more HIMARS/M270's would be more streamlined since they're already familiar with the systems and have the supply chains setup. It won't have to take months to do anything.
Same with NASAMs air defenses. Finland only has a half dozen of them. Norway and NATO could get them those in a matter of weeks, not months unlike with Ukraine where there's months needed for training. Same with 155mm howitzers and self propelled guns, radars, Harpoon and RBS variant antiship missiles, and up to and including things like HARM missiles, F-18 jets (and F-16/F-35 jets), Leopard tanks, and all sorts of IFV's, APC's and MRAPs.
Plus think of what Sweden and Norway and Denmark and Poland gave to Ukraine rather readily a month or more after the conflict. That could all go to Ukraine instead and likely be more substantial as well. Starting in late March (so assumingly after Ukraine fell) Sweden gave Ukraine 10,000+ AT-4 missiles and RBS-17/Hellfire Missiles as well as loads of other military equipment. Norway sold the US 8 NASAMS systems to be sent to Ukraine, along with 3 M270's to UK so they could send three to Ukraine, and 22 M109 155mm SPG's, 5000 M72's, and loads of drones etc. Even Denmark sent 2700 M72's, 300 Stinger MANPADs, and a few dozen M113's.
Plus the volunteers that might want to take part in the Special Military Operation.
It'd be extremely rough for Finland since I think Russia would actually be more ruthless than they have been against their "little brother' Slavs in Ukraine. They'd probably be striking infrastructure and population centers far more ruthlessly IMHO to try and demoralize the Finns and woe be to any towns or villages or cities captured by Russian forces as well, especially after realizing it won't be a cakewalk.
