Russia’s invasion of Ukraine has been framed in the West as a key flash point in a wider ideological struggle against militarism and dictatorship. Yet because of the risk of global nuclear war, Western democracies cannot risk direct participation. In these circumstances, the stage has been set for foreign volunteer fighters to play a prominent role in this conflict.

On Feb. 27, Ukrainian President Volodymyr Zelensky announced the creation of an International Legion within the Ukrainian territorial forces, waiving the need for visas, inviting applications through its embassies and publishing step-by-step instructions for potential volunteers. Various foreign governments have indicated that they would look favorably upon such volunteers, and media reports indicate that thousands of people may have already enlisted.

Zelensky’s call for foreign volunteers has precedents, most notably the foreign volunteers who participated in the Spanish Civil War. Their history offers insight into what prospective volunteers — and the world — should expect. While there are practical issues and drawbacks to consider when inviting foreign recruits, such volunteers have made significant contributions in the past, both as fighters and enduring symbols of global solidarity.

In 1936, a fierce civil war raged across Spain and armies fought to control the capital of Madrid. As the elected Republican government fought Francisco Franco’s rebellious Nationalists, a column of foreign volunteers arrived to bolster Madrid’s defenders. Such units became known as International Brigades, eventually encompassing tens of thousands of foreign volunteers fighting to preserve Spain’s fragile yet inspiring democracy. In the words of British communist politician Harry Pollitt, they were “fighting for the democracy not only of Spain, but also of Britain and all of Europe.” All democracies, he asserted, should “support and strengthen” those confronting fascism on their behalf.

Similar rhetoric fuels the global response to today’s crisis, as Ukrainian authorities frame the conflict as an existential threat far beyond Ukraine’s borders — in Zelensky’s words, “If we will fall, you will fall.”

An exceptional number of foreigners volunteered to fight in Spain. Such recruitment was possible not only because of mass sympathy for the Spanish Republican cause, but also because that sympathy was deeply rooted in existing anti-fascist movements and particularly the transnational networks of international communism.