In any military campaign, geography is key. Ukraine shares a 1,900-kilometer (1,200-mile) border with Russia. Ninety percent of Ukraine is flat terrain, punctuated with small hills, depressions and shallow riverine valleys. It is excellent ground for armored warfare, and Ukraine's border is not anchored to any specific or defensible geographic feature.
The Dnieper River is a key terrain feature, sloping from the north of Ukraine into the southeast before switching back west of Crimea to the Black Sea. The river effectively bisects the country into its eastern and western regions. The Dnieper River Valley has a well-developed network of ravines and gullies, extending into a system of tributaries.
The terrain in eastern Ukraine is predominantly steppe (Ukraine's grasslands, mainly in the south) or forest-steppe (historically broadleaf forests and grassland in a band across Ukraine's center). This provides very good observation across the flatlands. Anyone or anything cresting the high ground would be observable for kilometers. Large formations moving over any kind of distance would find it nearly impossible to conceal their movements. The best cover from direct-fire weapons would be the natural protection of slopes and terrain features. The open terrain provides little cover from the air, offering a huge advantage to whoever controls the skies.
Although an actual invasion of Ukraine by Russia is highly improbable, if it were to occur Russia likely would not stop until its forces came to the Dnieper River. Military planners would have to assume that there could be a counterattack on any territory they absorb, necessitating a security presence for an indefinite time. Anchoring a defense on a terrain feature such as the Dnieper would help make this territory realistically defensible. Some of the major tributaries in the northeast off the Dnieper could also serve as anchors if the Russians decided not to drive across the northeast all the way to Kiev. However, small territorial grabs in the east or extending westward beyond the Dnieper toward Moldova along southern Ukraine would place Russian lines in wide-open terrain that would require a large number of resources to protect and maintain.
Obstacles and Priorities
Although eastern Ukraine lacks geographic obstacles, the time of year is important; the spring thaw has begun after a mild winter and recent rainfall has already made the ground extremely boggy. Vehicles, even tracked ones, could quickly become mired off-road. The most defined obstacles are the myriad river systems and accompanying gullies. However, several highways cross these rivers.
Three features would be critical for any offensive or defensive campaign in Ukraine. The first would be any bridges or roads spanning restricted terrain. The second would be any military — or possibly civilian — installations capable of receiving or deploying manpower or materiel, such as bases, ports or airfields. The third would be any large urban centers that could shelter an enemy force and serve as a possible transportation or supply node. Urban centers can become significant problems for military planners, especially if they contain a hostile population, and would be treated as obstacles. Blocking key avenues of approach is difficult because almost any position can be flanked or circumvented, assuming environmental conditions permit off-road movement.
Russia's Order of Battle
As of January 2013, 766,055 officers and soldiers were paid to serve in the Russian armed forces. Approximately 400,000 of these serve in the Western and Southern Military districts, which directly border Ukraine. There are also approximately 22,000 Russian troops in Crimea. These forces are bottled up because of the narrow land bridge connecting Crimea to Ukraine, but they could be moved by air or sea.
Given Russia's vast borders, only a portion of its overall forces could be tasked with an invasion of Ukraine. The Russians must continue to maintain a significant presence in the North Caucasus, using manpower traditionally provided from the Southern Military District. To maintain a military presence along its western borders with NATO and Finland, Russia leans heavily on forces within its Western Military District.
For a scenario as complex as an invasion, Russia would most likely draw on principal units from the 20th Army of the Western Military District, based out of Voronezh, and elements of both the 49th and 58th Armies of the Southern Military District, headquartered in the North Caucasus. The Russians could also rely on a number of special operations forces, such as the Spetsnaz and Vozdushno-desantnye voyska airborne units, although a number have already been deployed in Crimea. Of the 5,000-10,000 special operations forces in Russia, a portion could easily be redeployed by air to where they were needed.
Ukraine's Order of Battle
The Ukrainian military consists of approximately 130,000 personnel, of which around 65,000 serve in the ground forces. Of these troops, only between 6,000 and 10,000 are fit for service. Moreover, less than 20 percent of Ukraine's armored vehicle crews are sufficiently trained. In Ukraine's air force, only 15 percent of Ukrainian combat aircraft are serviceable and only 10 percent of its aircrews have trained for combat. The Ukrainian government is setting up a new national guard that could supplement these troops.
The vast bulk of Ukrainian forces are based west of the Dnieper River — a legacy of the Cold War, when Soviet forces were positioned for a fight against NATO. The only brigades east of the Dnieper are the 1st Tank Brigade to the north, the 79th airmobile brigade near Crimea, the 92nd and 93rd Mechanized Brigades in the east, and the elite 25th Airborne Brigade near Novomoskovsk. Any troops relocated from Crimea would need to be rearmed, though they could be assimilated into Western forces.
Russia's troop presence on Ukraine's border is substantial and growing larger, with primary concentrations in the Kursk, Belgorodskaya and Rostov regions. According to U.S. military and NATO sources, the Russians have positioned approximately 30,000 troops along main roads leading to the border. These troops are at a high state of readiness and are well supported by air defense and logistics. The logistical buildup in particular has raised concern in Kiev and the West because it indicates that Russia could be planning for sustainable operations. It also diminishes any early warning because there are reportedly sufficient resources in place to initiate a strike into eastern Ukraine without additional support.
According to official sources, U.S. military intelligence has reason to suspect that Russia has already deployed elements of the 45th Spetsnaz regiment, affiliated with the Russian Main Intelligence Directorate, or GRU, in eastern and southern Ukraine. Likely tasks for strategic special operations forces would include the surveillance of local Ukrainian military deployments, coordination with local sympathetic forces — similar to what has occurred in Crimea — and preparation for possible acts of sabotage in the event of a Russian invasion.
Ukraine is surrounded by Russian forces and their allies, including forces in Transdniestria and Belarus, and in a low state of readiness. The Ukrainian military has had to rely disproportionately on its few well-trained or elite units as it responds to crises around the country. For example, a large number of troops from the 25th and 79th Brigades have been moved close to Crimea, forcing the military to rely on other troops to patrol eastern Ukraine.
Kiev has embarked on a major effort to mobilize its troops and improve readiness. Ukrainian forces, likely from the 1st Tank Brigade, have already been sighted closely patrolling the Russian border in Strilecha. Other Ukrainian forces in the east are showing their presence around the city of Donetsk, and the newly formed national guard can also be counted as an opposition force.
A Potential Russian Advance
Given the favorable terrain, infrastructure and proximity of key objectives in eastern Ukraine, a Russian advance would likely consist of rapid thrusts using tanks and armored infantry fighting vehicles at several points across a broad front; Russian brigades are geared toward fast and violent onslaughts. From the areas where Russian forces are concentrated in the regions of Kursk, Belgorodskaya and Rostov, the Russians can invade along numerous highways.
The Russians would advance with reconnaissance elements leading the way to identify any enemy locations and dispositions. Long-range heavy artillery and combat aviation would be used to engage any fixed targets or hardened points. Urban strongholds would initially be cordoned off and bypassed, though at some stage Russia would have to pacify any towns, villages or cities. Securing urban populations and protecting supply lines would present a significant manpower burden for the Russians.
Although weak, Ukraine's air force still poses a threat to any invading force. It would need to be neutralized, particularly if the Russians were to carry out significant airborne operations — likely to secure key infrastructure ahead of an armored advance — or paratroop drops into the natural drop zones provided by Ukraine's open spaces. Although the time frame is difficult to calculate, the Russian air force could achieve air superiority in 24-48 hours. This would require the coordinated destruction of strategic Ukrainian air defense and radar installations and strikes against Ukraine's Su-27 and MiG-29 air bases.
In any scenario, Kiev would become a nexus for defense. A rapid thrust by Russian forces toward Kiev would pin down substantial Ukrainian forces, enabling Russian ground assets to maneuver elsewhere and overwhelm Ukraine's dispersed eastern military elements. Given distance, terrain and force considerations, the Russians probably could take Kharkov and Luhansk quickly, even if resistance continued in the urban centers. Donetsk and Mariupol would take a little longer before succumbing. It is unlikely that Russia would seize Odessa as part of the initial thrust, but the Black Sea Fleet would be able to blockade the port, preventing the Ukrainian navy from being brought into play.
The first serious geographical obstacle that the Russians would encounter is the Dnieper, located between 200 and 300 kilometers west of the Russian border. By the time the Russians could reach the river, Ukrainian forces would have had time to destroy key bridges. Assuming the Russians have air supremacy, any amassed defensive forces along the Dnieper's western bank would suffer heavy attrition from Russian aircraft.
At this point, the Russians would have seized the Russian-speaking majority areas of Ukraine. If Russian forces were to continue farther west from the Dnieper, they would probably need a strategic pause to consolidate their position, deal with pockets of resistance, resupply and prepare for bridging operations.
Resistance and the Difficulties of Holding Territory
Overall, the Russians are quite capable of effectively and rapidly seizing a considerable portion of eastern Ukraine. Such an operation would be significantly different from Crimea, however, in that the Ukrainian forces and general population likely would offer far more resistance. Russian casualties, even in a generally successful operation, would be substantial. While the Ukrainian military is no match for Russia in a head-on fight, forces in the east would most likely fight a delaying battle, trading space for time as they withdrew, hoping to harass, pin down or cause attrition to advancing Russian forces. Small, dispersed pockets of Ukrainian fighters could achieve a disproportionate effect if they used the ground effectively, especially in built-up areas. Likely tactics would be ambushes, mining approach routes, installing man-made obstacles and destroying bridges on approach routes.
Any formalized resistance would be short-lived, with Kiev turning quickly to the West for assistance. However, the enduring Russian cost for invading Ukraine would be significant, particularly since the West would not simply allow Russia to consolidate its border with NATO and Europe by way of invasion.
Russia would have to deal with the internal problems in this newly occupied area as well. This could be particularly difficult because though much of eastern Ukraine's population is pro-Russian, they are not necessarily separatists like the Crimeans or eager to join Russia. Any direct invasion of eastern Ukraine would turn much of that population against Russia's intention. These sentiments could be facilitated and supplemented by small-scale resistance from militant groups and the newly formed national guard. In addition, any Russian invasion of eastern Ukraine would disrupt the economy in that region — Ukraine's industrial heartland — and thus aggravate the population's hostility toward Russia's presence. Also, the eastern Ukrainian economy is tied to the economies in Russia's western regions, so the economic impact would ripple into Russia.
As Iraq proved for the West, it is relatively straightforward to invade a country, but it is much harder to keep it. Moscow would have to cope with a focused and protracted insurgency and guerrilla-style fighting. Questions remain about how far Russia will go to prevent Kiev from aligning with the West, but a full invasion of Ukraine could be a literal and figurative bridge too far.