As Russian forces occupy Crimea—and especially if they invade and hold wider swaths of the country—Ukraine and the West will face momentous decisions about backing an insurgency. The Kremlin may fear the toll that insurgents would take on occupying forces. The West should reinforce this anxiety.

Kremlin leaders profess an interest in negotiations, yet tens of thousands of Russian soldiers and airmen are arrayed along Ukraine’s eastern and northern borders. With little tactical warning they could unleash an assault. The buildup is somewhat akin to those before Russian forces swept into Chechnya in the 1990s, and into Georgia in 2008.

Moscow’s intentions toward Ukraine remain uncertain. Crimea may be enough, or Russia may pursue a wider conquests such as :

1. A land bridge across southeastern Ukraine to Crimea

2. Eastern and central but not western Ukraine, or

3. All of Ukraine.

Blazing a land bridge to Crimea would require massing Russian troops only on the southeastern border of Ukraine. A land bridge would facilitate Russian economic and military ties with Crimea.

Yet Russian forces are also poised across Ukraine’s eastern and northeastern borders. This suggests the Kremlin is contemplating taking eastern and perhaps central Ukraine. If Kyiv were seized, Ukraine’s government would be forced into exile, perhaps in the western region of the country.

The third option would incur higher risks—anti-Russian sentiments in western Ukraine are strong. On the other hand, an occupier's scorched-earth tactics are more effective when all the contested ground is held. Ukraine would have to locate its government-in-exile abroad, perhaps in Poland.

The second and third options jibe with Putin's expressed view that Ukraine “is not even a country.”

Russia’s 2008 invasion of Abkhazia and South Ossetia caught Georgia by surprise. Its army was not mobilized and its president was out of the country. Russian forces had just completed a field exercise over the mountains in the North Caucasus, practicing the invasion of a South Caucasus country. Ill-prepared but motivated Georgian troops still managed to down several Russian combat aircraft and bloody some ground forces. Since then, Georgia has not mounted a notable insurgency against Russian occupiers. Ethnic Georgians were earlier expelled from Abkhazia and South Ossetia, making an insurgency there difficult to sustain.

A month ago Russia launched an even more successful strategy. In Crimea, Ukrainian troops were so surprised and weak that they did not fire a shot. It is unclear whether Ukrainians and Tatars in Crimea—over a third of the population—will conduct insurgency operations, or whether Kyiv would support this step. Many Crimean Ukrainians and Tatars may emigrate to other areas of Ukraine, and some Tatars will go to Turkey. North Caucasian insurgents might seek to stir Tatar resistance; to lessen this risk Moscow is sending emissaries to the Crimean Tatar community from its puppet regime in Chechnya.

In eastern Ukraine, Kyiv would have strategic warning of an attack that was unavailable in Crimea. Some defenses are prepared. Even without tactical warning and despite limitations in arms, training and mobility, Ukrainian forces could damage Russian invaders. By using selective defense tactics, even outnumbered Ukrainian troops could impose significant casualties. Moscow would employ cyber attacks on Ukraine’s command and control systems.

Ukrainians could not prevent Russian capture of large swaths of their country, but later could mount a strong and sustained insurgency. It would be indigenous and supported from unoccupied areas of Ukraine. Poland and others in NATO might support the insurgents. Younger Ukrainians officers tend to see Russia as a potential enemy. The wider the Russian occupation, the more support an insurgency would draw from the West.

Ukrainians would face daunting risks if they chose insurgency. Soviet and Russian counterinsurgency tactics are not about winning hearts and minds. They are about graves and mass punishment. From their occupation of East Germany during and after World War II, to Hungary in 1956, to Afghanistan in the 1980s and to Chechnya more recently, Moscow’s legions have employed rough measures. A Russian army reacting to an insurgency could visit horrible retribution on resistors and those around them. Europe could witness human misery not seen on its continent since Yugoslavia broke up.

Ukraine does not lack for experience in armed resistance. During and after World War II Ukrainian insurgents, at times numbering in the tens of thousands, fought both the Nazis and the Red Army in a struggle to create an independent Ukraine. After the war the West trained some Ukrainian refugees to be dropped back home, but the notorious British traitor Kim Philby foiled the plans. Current Kremlin propaganda is heightening awareness of Ukraine’s legacy of impassioned resistance.

By leveraging technology and tactics honed in the Afghanistan and Iraq wars, a new generation of Ukrainian insurgents could conduct a potent insurgency. Sophisticated improvised explosive devices (IEDs) could take a heavy toll on plodding Russian occupiers. The Kremlin may not appreciate the vulnerability of its troops. The Russian army is largely conscript and morale is not high, and its electronics and vehicles are not well suited to counter IEDs.

The West has declined to provide Ukraine with lethal defensive weaponry, such as antitank and antiair munitions. If Russia were to occupy parts or all of Ukraine proper, the surviving government may make a more persuasive case for weapons and for Western training and assistance to insurgents. The West has language-proficient paramilitary experts with good knowledge of Russian military and political tactics. They could improve the effectiveness of a Ukrainian insurgency.

If Russia were to seize parts of Ukraine beyond Crimea, and face an insurgency and wider Western sanctions, the Kremlin might eventually conclude—as did Gorbachev's Kremlin in the late 1980s—that occupying a resisting neighbor is too costly. In 1989 the Kremlin withdrew Soviet forces from Afghanistan, but only after Gorbachev had begun to liberalize the political system, and after economic costs of and popular opposition to the war had become lodestones.

If Russia were to occupy wider areas of Ukraine, it might never cede the territory absent a robust Ukrainian insurgency, democratic development at home, and rising economic and foreign policy burdens from armed revanchism.

William Courtney was US ambassador to Kazakhstan and Georgia, and special assistant to the President for Russia, Ukraine, and Eurasia.