From what I know, it seems fascism takes advantage of certain traits in a population. Nationalism, unity, prosperity. Powerful leaders mobilize an entire nation by crafting a common enemy, almost an 'us vs them' mindset. Why is it then only relatively recently we have had such large fascist movements? Or was it always the case, but we just saw it as the norm, people battling and swearing loyalty 'for the king'?
Well a lot of this depends on what, precisely, is meant by fascism. Defining fascism, and nearly all political labels, is fraught with complexity and politicization, even in a scientific context. The definition of these kinds of terms is far too often tied to your personal politics. If a random person told me "fascism and socialism are the same thing", it's a pretty good guess they don't like socialism or left wing politics in general.
If by fascism you mean, "social forces/groups/tendencies/movements that violently and/or aggressively seek to create/maintain/strengthen divisions based on things like nationality, ethnicity, religion, etc." then you could likely see instances of it in all of human history.
If the definition is "revolutionary right-wing political movement that valorizes violence, war, and aggression, is a reaction to perceived ills of capitalist society that functions to redirect anger at the failures of capitalist society into extremely virulent forms of chauvinism, enabling the capitalist class to maintain its rule in a time of deep crisis, and increasing the levels of intervention of the state in private industry the economy in a manner that enhances the power and wealth of the capitalist class over the working class" Fascism can only exist after the development of industrial capitalism and the modern state-form. I'm not a historian but if asked to give a range, I'd say this definition would mean fascism can only have begun to exist at it's earliest in the 1700s, and I think that's a stretch. It's kind of like asking if someone from 14th century China was a metalhead. You can probably find all kinds of strong analogy and similarity, but no.
The definitions generally used in social sciences are closer to the latter in specificity to modernity and to many other things than the former. The former is so broad that you could pose plausible arguments that nearly every country is fascist. For example, they grant significant rights and privileges to citizens/subjects/etc., and not to non-citizens, and enforce this division violently, yet to the individual citizenship is an accident of birth not unlike race and often functionally nearly identical to race (not many black Japanese citizens!)
Edit: just to be clear I'm not actually posing that argument