Back in the Brexit years, the European Research Group of hard-line Conservative MPs christened themselves the “Spartans”. Perhaps they were drawn to the image of an elite warrior brotherhood, standing firm against overwhelming odds. Or perhaps they admired Sparta’s reputation as one of the most austere and uncompromising societies of the ancient world. Either way, they might have profited from a closer look at how Sparta’s story actually ended — and why.

Sparta guarded its citizenship with exceptional rigidity. Full political membership was reserved for those born to two Spartan parents, and even then only after passing through an unforgiving system of military training and communal discipline. Foreigners were periodically expelled under a policy known as xenelasia.

Over time, this inflexibility proved fatal. The number of full Spartan citizens declined dramatically, from roughly 8,000 around 480 BCE to perhaps little more than 1,000 a century later. Military losses played a part, as did growing inequality in land ownership, but the core problem was structural: citizenship was so restricted that the ruling class steadily withered. A society that defined itself by exclusion gradually deprived itself of resilience.

There is an uncomfortable parallel here for modern nations confronting demographic decline. Across much of Europe and East Asia, birth rates have fallen well below replacement level while populations age rapidly. Nationalists insist the answer lies in boosting native fertility. Yet the evidence suggests this is far easier said than done. Hungary, under Viktor Orbán, has devoted vast public resources to pro-natalist policies. While these measures may have shifted the timing of births, the overall fertility rate remains well below replacement. Even generous subsidies cannot easily reverse deep social and economic trends.

Meanwhile, the demographic arithmetic grows harsher. Fewer workers must support more retirees. Pension systems strain. Health and social care costs rise. Tax bases narrow just as demand for public spending increases. Even if birth rates rose tomorrow, it would take two decades before those children entered the workforce. Time is not on the side of demographic nostalgia.

This is where immigration becomes not a slogan, but a necessity. Migrants are disproportionately of working age. They expand the labour force immediately, contribute taxes, and help sustain the very public services that ageing societies rely upon. They staff hospitals and care homes, build homes and infrastructure, start businesses and fill skills shortages. In towns and regions facing population decline, new arrivals can keep schools open, high streets viable and local economies alive.

Societies that resist immigration — whether Hungary, Russia or Japan — face an uncomfortable reality: without inward migration, demographic contraction becomes almost inevitable. The choice is not between change and stasis, but between managed adaptation and unmanaged decline.

For Britain, this means thinking strategically rather than nostalgically. International students educated in our universities represent a long-term investment in human capital; encouraging them to remain strengthens our economy and research base. Rebuilding closer ties with the European Union — including a return to freedom of movement — would reconnect the UK to a deep pool of skills, entrepreneurship and cultural exchange.

None of this is to deny that immigration must be managed well. Integration, housing supply and infrastructure investment matter enormously. But these are questions of competent governance, not arguments for demographic retreat. The alternative — shrinking workforces supporting expanding elderly populations — poses far greater risks to prosperity and social cohesion.

Sparta chose purity over adaptability. It defined belonging so narrowly that it diminished itself. Modern liberal democracies face a different choice. In an interconnected world, strength lies not in rigid exclusion but in confident openness. History’s lesson is not that identity must dissolve, but that societies unwilling to evolve rarely endure.

The consequences became unavoidable for Sparta in 371 BCE at the Battle of Leuctra. There, Sparta’s elite warriors were finally defeated by the Sacred Band of Thebes — 300 elite soldiers organised in pairs of lovers. A fitting final metaphor for the triumph of diversity over conservatism.

* Barry Smith is a member of Merton Borough Liberal Democrats.