Russian Mobilization in the Twenty-First Century
Since the publication of my guide calculating how many battalion tactical groups (BTGs) the Russian Armed Forces could generate against Poland, a couple critiques have been levelled at whether this is the best way to understand how Russia would fight Poland and/or NATO at large. My guide was intended only to demonstrate how many of Russian Defense Minister Sergey Shoigu’s declared 168 BTGs  were available to mass on the Polish border and has been taken somewhat out of context. Nevertheless, some of the critiques raise important points worth addressing.
The most important is the nature of Russian mobilization today, especially after the reforms started in 2008. The nature of mobilization has changed dramatically since the Soviet era, as Andrew Monaghan has described at some length.  Mobilization since 2008 is far greater than shifting men with guns, instead primarily optimizing the Russian government for conflict both administratively and operationally. In both the August 2014 September 2015 Russian military interventions, no Soviet-style mobilization of troops preceded the action. Though considerable Russian forces were moved to the Ukrainian border in March 2014, seemingly in anticipation of a classically styled invasion, these were largely (but not entirely) withdrawn over the following months. Instead, isolated artillery strikes from over the border numbed anticipation that anything more significant than combat support for officially non-governmental separatists would come from Russia.
It should be noted that the “100 BTGs” buildup of Russian forces on the Ukrainian border is US intelligence’s projection of a Russian war plan and does not yet reflect the reality on the border. Russian presence on the Ukrainian border approximates roughly 50-60 BTGs: 15-20 deployed from elsewhere  and nearly 40 local forces from the 20thGuards and 8th Guards Armies. However, this massing of forces has attracted so much attention that it should be read as a signaling exercise and not an actual prelude to war. If this buildup spills into conventional war, it would break with Russian practice in the 21st century; far more likely would be a withdrawal of troops and subsequent attack beginning with local forces and then deployment of other forces after the fact to hold territory.
As of 2021, Russia has three sources of manpower for the conduct of a military operation: the contractor force of the Russian Armed Forces, the BARS reservist force, and Rosgvardiya or the Russian National Guard. Russia’s conscripts no longer feature prominently in military operations. Conscription remains as a means of identifying talent in the youth population and for filling out necessary numbers for more perfunctory billets in the Russian Armed Forces.  Of these three forces, Rosgvardiya, possibly assisted by the Ministry of Emergency Situations, will be responsible for occupations. Russia only published some details of the BARS reservists’ organization this year during Zapad-2021  and so its role is not yet fully clear, though its primary purpose is likely to be rear area operations and unit replenishment. The Russian Armed Forces’ contractor force will take the lead in offensive and most defensive operations and has adopted the BTG as a standard unit not for its flexibility as some have suggested but because of its ability to operate in isolation. 
Moreover, the BTG is not as flexibly sized a unit as has been suggested. In Zapad-2021, the Russian Armed Forces started trialing “mobile tactical groups”  as more adaptable units seemingly as an experiment in different-sized units, albeit primarily to facilitate integration of heavy indirect fires into isolated maneuver units. BTGs are required for presence;  ground- and air-delivered intelligence-directed indirect fires are anticipated to be the primary strike capability of the Russian Armed Forces, thus the ability to rely upon mere battalion-sized units rather than brigades or other larger groupings.
Using a military district- and army-centric system for determining total BTG generation reflects that Poland, as part of NATO, would be unlikely to be completely isolated in a war. Though I ultimately found that only 64 BTGs would be likely available to fight Poland, the total number of BTGs Russia has available – including Southern and Eastern Military District assets as well as those Western and Central Military District assets unlikely to face Poland directly – reaches 150-175 depending on how to count certain exercise patterns, i.e. almost exactly as Shoigu claimed. Even if Russia successfully isolated a war to Poland alone, certain Western Military District BTGs would be needed to screen Norway, Finland, Estonia, Latvia, Lithuania, and Ukraine. The Southern Military District would be almost fully devoted to Black Sea region security given the possibility of NATO horizontal escalation in this region. Imagining that more than 50% of Russian BTGs would be allotted against Poland would be difficult to rationalize in this environment. Mobilization of other manpower options would not primarily to generate more BTGs as these are composed of professional contractors; additional mobilization would primarily serve to offer means of consolidating BTG- and fires-led gains.
Poland’s defense remains easier than Ukraine’s because Russia must screen against more potential adversaries because of Poland’s NATO membership and because deploying troops in any substantial number would require deploying through Belarus. However, those observations also underpin key points that Poland must prepare: because we assume that Russia would need to redeploy significant forces to Belarus to mount an attack, how would Poland respond to an initial non-contact air- and missile-delivered strike if there was no warning buildup in Belarus? Would it counterattack into Kaliningrad Oblast if there was no border infraction beyond missile launches?
Calculating BTGs in peacetime is necessarily hypothetical but, under current conditions, more useful than alternative methods of imagining Russian preparations for a possible war with Poland. The dispersed nature of forces in both Zapad-2017 and Zapad-2021 suggests that Russia anticipates both defensive and offensive military scenarios in Europe require dispersed units operating independently. Russia will not dump all assets across military districts to fight a single country in NATO: the entire point of the force buildup since 2014 has been to reduce the necessity for this.