Israeli soldiers take up positions close to the Israeli border with the Gaza Strip on July 22. (Abir Sultan/EPA)

The deaths on July 20 of two Americans serving in the Israel Defense Forces (IDF) provided an opening for critics of Israel to compare them to the foreign fighters of the Islamic State, formerly referred to as the Islamic State of Iraq and Syria (ISIS). Similar complaints have already called for other governments to criminalize volunteering for Israel to create equivalence to the prosecution of would-be jihadi Islamists. The IDF reports 4,600 foreign “Lone Soldiers” currently serving, over one-third of whom are American (it is unclear how many hold dual citizenship). Are IDF Lone Soldiers comparable to al-Qaeda-inspired jihadis or the volunteer brigades who joined the Spanish Civil War?

The question hinges on both definitions and connotations of what a foreign fighter is. Consideration of foreign fighters by international security analysts is less than a decade old and, as political scientists inevitably do, researchers employ slightly varied definitions, so there are no universal criteria for identification. Crucially, however, most studies have assumed foreign fighters to be insurgents fighting against the government. Scholar on Islamist militant groups Thomas Hegghammer’s definition of foreign fighters specifically “excludes returning diaspora members,” and this would encompass Lone Soldiers such as Nissim Sean Carmeli, an Israeli-born Texan who was one of the Americans killed. No published academic definitions of foreign fighters would therefore include diaspora Jews fighting in the IDF.

Beyond definitions, the term “foreign fighter” generally carries an implication of illegitimacy. In late 2001, al-Qaeda’s “foreign fighters” were shipped off to the Guantanamo Bay detention center in Cuba because they were regarded as both uniquely dangerous and uniquely “unauthorized enemy combatants.” They did not uphold international norms of citizenship and military allegiance, and stated that they in fact wished to destroy the international system itself. They were also not the primarily profit-seeking mercenaries already banned under international law.

Nearly every academic study has focused exclusively on Sunni jihadis, some incorporating Islamism in their parameters, although counterterrorism and Middle East security expert Daniel Byman has recently examined the substantial number of pro-regime Shiite volunteers who arrived in Syria from elsewhere in the region. The phenomenon is far wider than just Islamists, however. In my book “Foreign Fighters,” I analyze the surprisingly common strategy of armed groups that persuade volunteers abroad that they have a duty to protect fellow members of a transnational group facing a threat to its survival. This approach has been used by ideological affiliations including the Communist International for the International Brigades, and by religious groups like the Catholic foreign fighters on the other side of the Spanish Civil War, who were told their souls would benefit from martyrdom for Christ. It also holds for ethnic groups like the nearly 200 Albanian-Americans who joined the Atlantic Brigades to fight for the Kosovo Liberation Army, and the European volunteers defending fellow White Anglo-Saxon Protestants in the Texas Revolution who outnumbered Texan-born fighters at the Alamo three-to-one. The particular identity of the group does not affect this approach to recruitment. Today it is being used by jihadis for the Islamic State and also pro-Russian fighters for the Orthodox Dawn in eastern Ukraine.

A better comparison is not today’s “Lone Soldiers” but the recruitment drive launched in 1945 by David Ben Gurion, later Israel’s first prime minister, to obtain the assistance of diaspora Jews to rapidly build a regular military. Anticipating an invasion by the armies of Arab states in response to a declaration of statehood, Ben Gurion turned to North American donors to equip and field experienced World War II combat veterans in a movement later known by the Hebrew acronym MACHAL. Their pitch was not the opportunity for a Jewish state, but the inevitable resumption of the Holocaust without one. Over 5,000 volunteers came from over 40 countries and they, particularly the pilots, were subsequently credited by Israeli leaders from Benjamin Netanyahu to Yitzhak Rabin as having played a decisive role in the outcome of the war. The president of the MACHAL veterans told me that, had there been 20,000 of them, “there never would have been an East Jerusalem.”

Over the next 50 years, diaspora Jews, including Chicago Mayor and former White House chief of staff Rahm Emanuel, volunteered for IDF roles during security crises. In the late 1990s, the Mahal2000 program encouraged Jewish youth from abroad to serve in combat units for stints of 14 months. The IDF today incorporates this successor program as one option for service by non-citizens. Thousands of Western Jews have now volunteered, sought by the IDF to fill manpower gaps, resulting in what has been termed the “Diaspora’s Foreign Legion.”

The legal status of this sort of foreign fighter has long been open to debate. The United States banned its citizens from becoming foreign fighters while George Washington was still in the White House, despite its reliance on foreign fighters like the Marquis de Lafayette during its own revolution. Other famous volunteers, nationalists such as Tadeusz Kosciuszko of Poland, and Italy’s Giuseppe Garibaldi who led rebels in Brazil, demonstrated the threat of blow back against home governments by returning militants long before the birth of the jihad movement. Today nearly every country has some rule against foreign military or paramilitary participation.

But these rules are as varied as academic definitions of foreign fighters. The United States, for example, bans foreign military recruitment, revokes citizenship for Americans who become officers of foreign militaries, and occasionally prosecutes individuals for joining groups determined to be working against national interests. But there is no penalty for serving in another country’s military or in a rebel group deemed non-hostile to the United States, and particularly that of one deemed to be an ally. Additionally, there could be a legal dynamic with dual American-Israeli citizens, who are required to serve as Israelis in the military, though that has not yet become a concern. Like the French Foreign Legion, programs sponsored by the Israeli Ministries of Defense and Absorption to attract and serve the needs of Lone Soldiers are run online or in person from Israel, so they do not fall afoul of the sovereignty of other states. The diaspora Jews who become IDF volunteers are convinced that Israel is uniquely singled out for de-legitimation and requires their contribution to its defense. If history is any indication, efforts to block such volunteering will only raise threat perceptions and increase volunteering, ensuring that the Lone Soldiers will not want for companionship.

David Malet is a senior lecturer in international relations at the University of Melbourne and associate director of the Melbourne School of Government.