Commanders of the Ukrainian armed forces and Western analysts recognize that given the major disparities in capabilities, operations by the upward of 190,000 Russian military and internal security personnel now deployed around and in parts of Ukraine will likely overwhelm the country’s conventional defenses. President Volodymyr Zelenskyy has ordered general mobilization, and Ukrainians are already engaging in irregular warfare and preparing for a prolonged resistance. What might a comprehensive Ukrainian resistance entail? What is the potential effectiveness, and what are the risks? What support could the international community provide?

Although the armed forces of Ukraine have about 209,000 active duty personnel with better training, experience and weapons than in battles with Russian forces around 2014-2015, they are heavily outmatched. Ukraine has also been increasing its reserve forces with several hundred thousand personnel, many with recent military service. President Zelenskyy called reservists to active duty on Feb. 22 to rapidly reinforce combat units and support regional defense efforts.

Nevertheless, a large-scale Russian military action could quickly seize much of the eastern part of the country. A Russian occupation would be much more difficult and costly. Elements of the armed forces could sustain a defense of unoccupied parts of Ukrainian territory and work with other military forces and volunteers to pursue an insurgency in areas under Russian control.

Armed resistance against an occupying power would not be new to Ukraine. During and after World War II, the Ukrainians resisted Nazi and later Soviet forces More recently, voluntary units played a role in Ukraine’s ability to respond to the military crisis in eastern Ukraine in 2014.

The law establishing Ukraine’s national resistance strategy only entered into force in January 2022. There is little time to implement the comprehensive resistance efforts the law envisions, given the ongoing Russian attack. The new law gave the commander of the armed forces authority to manage territorial defense through the commander the Territorial Defense Forces, or TDF, and the resistance movement through the commander of Special Operations Forces.

The TDF is tasked with providing immediate defense of the population and infrastructure, helping maintain civil order, supporting operations of the regular armed forces, and assisting in the formation of centers of resistance in case of an occupation. The TDF, if and when fully established, could add 25 regional, light infantry brigades with over 150 battalions. The core of the force is planned to consist of 10,000 active duty troops, expanding to 130,000 after mobilization. The bulk of the force would consist of voluntary formations of civilians, particularly those with military or law enforcement experience, willing to support armed defense of their country without joining the military force full time. Zelenskyy recently ordered military exercises for these volunteers.

About 400,000 Ukrainian personnel have served in the military campaign against separatists in eastern Ukraine. Those not still in the service who have returned to their communities provide a pool of experienced fighters who could support a range of resistance and unconventional warfare efforts. The Ukrainian parliament has authorized volunteer, paramilitary forces the right to use small arms, which are now being distributed, and personal hunting weapons. The immediate focus is to establish the territorial units in the areas on the border with Russia and Belarus. However, it will likely require many months for the TDF to achieve full operational capability.

In the second day of Russia's incursion into Ukraine, troops attempted to defend the capital as battles were fought on multiple fronts.

Another force engaged in conventional and resistance efforts is the National Guard of Ukraine. With authorized strength of 60,000, the NGU was created in combat conditions in the wake of Russia’s intervention in the Donbas region. While its mandate is to protect the country’s constitutional and territorial integrity and maintain public order and safety, one of the goals was to improve the coordination and government control of the voluntary militia units engaged in the fighting in eastern Ukraine.

Battle-hardened with improved weaponry and training, its armored, mechanized and light infantry battalions could be capable elements of Ukrainian armed resistance.

Since 2015, the U.S. and other Western militaries have been training various Ukrainian forces at the Yavoriv Combat Training Center in western Ukraine. Over time, the program evolved from direct training of National Guard units to mentoring Ukrainian instructors to train regular Army, Special Operations Forces, National Guard and naval infantry units with the goal of enhancing Ukraine’s self-defense capabilities, readiness and long-term force development. U.S. and other Western military trainers have been withdrawn from Yavoriv and repositioned elsewhere in Europe.

Ukraine’s president has announced general mobilization for 90 days. How many Ukrainian citizens would fight or support a resistance under a wide-scale occupation remains unclear. Although approximately one-third of the respondents of a recent poll said they could engage in armed resistance.

Opinion polls only capture a snapshot of time and may not withstand the reality of war. Yet, they and the reports on mobilization could be an indicator of the number of Ukrainians who might join the resistance.

Resistance-capability development and the actual fight may include a variety of risks and requirements, such as the need to ensure careful vetting of personnel to avoid insider threats and accidents; the need to maintain popular support; and the requirement for solid training and exercise programs for voluntary forces.

Armed resistance may also create additional risks during wartime: increased attempts by Russia to penetrate Ukraine’s military structures and sabotage operations, and the potential that armed resistance may increase the risk that the adversary may retaliate against civilians, or at least the civilian supporters of the armed resistance fighters.

Meanwhile the existence of numerous resistance groups that may not be coordinated and may lose the ability to communicate under electronic warfare effects, may increase the level of battlefield confusion and thus may hamper Ukraine’s ability to achieve its overall strategic goal.

While the Ukrainian government has taken steps to improve its control over volunteer militia groups by integrating them into state organizations, it is unclear how successful these efforts have been. Some groups have been previously surrounded by controversies, with reports about a lack of control by Ukrainian authorities, poor discipline, political extremism and even human rights violations. For example, the U.S. Congress in 2018 banned American assistance to the Azov Battalion due to the group’s far-right nationalist alliances.

In sum, Ukraine’s armed resistance fight will likely unfold in a complex and dynamic environment with operations by several military and paramilitary forces. Although the war has already started, Ukraine will have to make further investments in the preparation and training of civilians, either as mobilized forces or members of voluntary forces, to wage a comprehensive resistance campaign.

Previous research by the think tank Rand on historic aspects of armed resistance concluded that, overall, such efforts have limited chances of imposing decisive costs on a highly capable occupier, especially if the aggressor is committed to total victory. Our research also found that external military and political support, or the lack thereof, can have a decisive impact on the outcome of the conflict.

While Ukraine has considerable internal resources and human capacity to raise the costs of a Russian occupation, sustaining a national resistance would almost certainly require significant international support. This could include political support, economic assistance and military assistance in the form of providing weapons, communications equipment, intelligence and training.

Stephen J. Flanagan is a senior political scientist at the think tank Rand, where Marta Kepe is a senior defense analyst.

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