The Spanish Flu pandemic started in the very last stages of WWI. Historians have long determined that the war indeed factored in the insurgence and spreading of the pandemic. Unprecedented numbers of people moved with modern and faster transportations allowing influenza to spread worldwide at an alarming speed. 

Historians still debate where exactly the Spanish Flu originated. It had long been thought it arose in a faraway farm in Kansas (USA), though recent archival researches have shown that it probably arose in China. 
In any case, it is a fact that the mobilisation of millions of soldiers and not combatant labourers across the globe, both to reach the battlefields and then to go home after the Armistice, was a huge factor in the spreading of the pandemic. 

Infections in the trenches

In all wars previous to WWI, more soldiers had died of infections and diseases than of wound from bullets and shells. This may explain why doctors and nurses were so worried about infections of any kind and did all they could to stop them. 

Life conditions in the trenches were undoubtedly the perfect spreading ground for an infection like the flu. Soldiers were exposed to cold and dampness for long periods. Stress was rampant, both because of the bombardments and for the emotional strain characteristic of that life. Soldiers were also often malnourished, especially during the time in the frontline trenches. 

All of these are factors that cause the immune system to become weaker. 
Soldiers all lived cramped together, in a filthy environment, where wounds were great hazards. Even superficial wounds could get infected by mud or other diseases commonly endemic in the trenches.

It may seem surprising that an epidemic didn’t break out earlier, but statistical researches have shown that the Spanish Flu had a lower mortal rate among soldiers who had served in the trenches for longer periods of time. This suggests that in the years after recruitment, but before the pandemic outbreak, soldiers became progressively immunised by exposure to seasonal flu. 
Smaller flu epidemics probably occurred in the trenches often, but the Spanish Flu hit in a specific moment: the end of the war and the demobilisation of millions of soldiers. 

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Modern transportation and the flu

The Spanish Flu pandemic reached every corner of the globe, even far away places like Alaska and the Pacific Islands, where it was particularly deadly due to the vulnerability of people not accustomed to flu exposure. 
There’s no doubt that both the needs of the war efforts and the new, swifter means of transportation factored into the insurgence of the flu and its subsequent fast spread. 

This influenza infection hit the trenches more or less in the spring of 1918. Clustered emerged in different cities across Europe and North America in that time, probably brought over by soldiers on leave. But it was the Armistice that caused the most damage. 

The harsher wave hit in November-December of that year, precisely when, after the ceasing of combats, armies demobilised, and millions of soldiers and other war personnel went home. 

People all across the world were involved in the war in Europe in different capacities. These people had been exposed to the new influenza. Regardless of the slow demobilisation times, they reached their home quite swiftly, thanks to the modern transportations, and once home, they celebrated the end of the war. People crowded in the streets to celebrate or gathered in private houses, welcoming the soldiers home. 
It would be difficult to imagine more ideal circumstances for the spreading of a pandemic.  

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RESOURCES

National Geographic – History of the Spanish Flu Pandemic
The Wall Street Journal – Influenza
OpenLearn – What was the impact of ‘Spanish flu’ on the armistice?
The Conversation – World War One’s role in the worst ever flu pandemic

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