I have overlooked the map, the longitude of the borders between the Soviet Union and Manchuria, as well as Beowulf's force mobilization statistics and all that, and I have found the following:
1) he does not have enough manpower to even properly defend the border in peacetime (8 fortress divisions + 8 infantry divisions is not even nearly enough for that, and that's assuming none of these divisions defend any cities inside his territory but are all put along the border - very risky disposition). The general strategic thought of the time was that a division can maximally defend a front 20 km wide (stretched to the max).
Though technically you can spread divisions on 100-150 km wide fronts, that would result in extremely weak defensive density and of course, cannot be considered anyhow effective against breaches where
several divisions are concentrated on 20 to 80 km fronts.
Thus most nations actually considered point defence (i.e. creating fortified regions around cities, all that) and keeping forces mobilized at those cities and ready to move out. The border would not be permanently manned. However, Beowulf insists that it's manned, so I calculated it on that basis.
The total width of borders of his Heilongjiang province with Russia and Mongolia alone is more than 2500 km (only the Argun-Amur borders alone are 1500 km long), if I measured it correctly on Wikimapia (thanks to it's excellent measuring tool), and his Argun-Amur border with Russia alone is 1000 km long.
If he says that
sections of the border fortifications are manned by divisions, he should fully understand the implications of such stretching. 16 infantry and fortress divisions (that's all his fortress and infantry divisions available in the territory), plus 4 mechanized infantry divisions would man a border of 1500 km wide (the three Mountain divisions as I gather man his far North borders with me), one division per 90-100 km roughly. A 20 km section would thus be defended only
by a single regiment, at best. See calculations of his defensive thingies below:
2) His defence line, if considering approximately the proportions of the Maginot line - (Maginot line - 5000 blockhouses, 142 large fortifications, 352 casemates, 78 shelters, 400 km wide, thus giving a proportion of 90% blockhouses, 6,5% casemates and 2,5% large fortifications) and applied to a 4 times longer border (1500 km only to Amur's northenmost point) or a 6 times longer border if he also fortifies the North, is not much to talk about actually.
Applying the rates, that results in 2050 blockhouses, 148 casemates, 56 large forts (or fortresses) per 1500 km (let's say no one gives a crap about the Far North because frankly, it should be so), or - 1 blockhouse per kilometer, 1 casemate per 10 kilometers and one large fort per every 27 kilometers of border, of his first defence line (Line Yat).
Assuming 50% of them are old, because construction of such magnitude and on such an enormous territory can't be fully accomplished in 5 years unless Beowulf wrecked his economy, there's about 70 new construction casemates and around 30 new large forts along his entire border.
Note that there's also a line 50 miles behind it mirroring it's fortifications, so Beowulf actually built 4100 blockhouses, 300 casemates and 112 large forts, effenctively the same as the Maginot line or very close to it in terms of size. So I wasn't joking about him having the Maginot line, he actually wrote a system of fortifications roughly equal to it. Well, perhaps on a larger border, but the amount of construction does not change, and in fact it increases the larger the border.
He has 3000 km of purely military railways (one along the border and the other doubling it 10-20 km away) running along the border, considering that Soviet-Mongolian-Manchurian border stretches 1500 km wide. I don't think any nation had such shit in real life, and he should be either disallowed to have it, or penalized in economic terms.
3) There is simply no realistic way for him to seriously mess with my offensive in it's initial stages - there's barely a division's worth of men on the attack vectors, and that's me being generous and basically putting his larger fortresses manned by FDs (or a large share of that FD) where I attack (not exactly rational, I know). I'll post the map later today.
Map (preliminary) - only cadre forces displayed, date of the attack (doesn't display all cadre forces either, only rifle divisions, fortress and mountain divisions, I'll make amends to it later)
Think about it. There's also 30 divisions in reserve at my hand (10 Corps) and 9x3 = 27 reserve divisions at Beowulf's hand, and possibly around 2-4 Chinese divisions that he managed to prepare for battle.
I have 21 cadre rifle divisions, 2 territorial (50%) divisions, 2 tank "brigades" (oversized), 8 cavalry divisions + 30 reserve rifle divisions, i.e. 53 rifle, 8 cavalry divisions and two oversized TBs versus Beowulf's 20 cadre divisions (only part of them able to participate, since they man other parts of the border - fortress divisions cannot be moved at will, and with a 1500 km long border there's about one fortress division on each 180 km of border) and 27 reserve rifle divisions - 47 divisions, of which clearly not all are even close to the front line.
That's 297 000 men in rifle divisions, 20 000 in tank forces, 56 000 in cavalry forces + 405 000 in reserves immediately in the region. Beowulf has 120 000 in fortress divisions, of which only 15 000 on each flank can be active (re-dislocating them would be a nightmare, with all the supplies stored in fortifications), plus 120 000 in rifle divisions and 60 000 in motorized divisions, and 405 000 in reserves. He is defending a 1500 km wide border and his mobile forces are twice less than mine.
I'll make maps with battle progression and all logistical information quite surely. Never thought it would come to this, but...