Working mothers have never had it easy — but a century ago this weekend, they won a landmark victory. On Nov. 29, 1919, the nascent International Labor Organization adopted the Maternity Protection Convention of 1919, calling for 12 weeks of paid maternity leave, free medical care during and after pregnancy, job guarantees upon return to work and periodic breaks to nurse infant children. In the decades since, every developed country in the world has established or surpassed this international standard — except the United States.

How did this coalition of government, industry and labor leaders come to recommend such a forward-looking policy a century ago? It wasn’t by choice. Instead, it was the result of feminists and female trade unionists demanding fair labor standards for working women — including paid maternity leave — as a matter of social justice and international security in the aftermath of the First World War. Global leaders bent to their will.

Women played a vital role in the war. The conflict mobilized whole economies as well as vast armies; as working-age men dug into the trenches, women took their place on the factory floor. Poorly paid seamstresses and domestic servants laid down their needles and aprons and went to work building tanks and filling shells. War work was grueling, but it paid decently, and female laborers took pride in doing their part.

Most female munitions workers were young, single and childless, but a sizable minority were married and in their peak reproductive years. The high demand for female workers made the delicate balancing act between productive and reproductive labor a priority for warring nations. In some English factories, pregnant women were moved to lighter tasks to keep them working up to delivery. In France, the legislature mandated factories provide on-site nurseries and paid breaks for breastfeeding mothers. Women flocked to trade unions in record numbers.