Setting: fantasy world in which non-magic users outnumber magic users, but are still very fearful of them, to the point of fighting a war to keep magic out of the country the story is set in. The whole story is based on the outcome of this war, as it begins directly afterward. Question: What is a plausible reason for why the magic users don't win against the human army? Is it enough to say that they were eventually overrun and repelled by the mundane soldiers, due to the disproportionate populations?
There are many possible reasons feel free to pick any you like
Magic is a poor weapon - arrows fly faster and more accurately than fireballs
Magic users have a code of ethics that keeps them from fighting well or at all - wizards all take an oath never to kill
Magic is good but there are too many muggle troops - each mage killed one hundred before they died, but there still remained thousands of foes.
Poor tactics - many times a more powerful army can be defeated if caught be surprise or in a bad condition, maybe there was an ambush were many mages were killed by surprise.
Magic users may want to move to that country, but they ain't gonna fight a war over it. Why would they want to live somewhere where people are hostile to them?
On the other hand, the mundane people are protecting their way of life, and would go to war.
So no war is fought, or if one is fought, its only a faction of the magic users that fight. The mundane folk would fight very hard against them, being more motivated.
Sure it's possible, as long as the mundanes can produce more soldiers then the magic user can kill. If each magic user can kill a hundred people before they die, then the mundane soldiers must have the population to produce a hundred fifty troops for every one magic user on the field.
Alternately you could give you mundane advanced technology that can even the odd between them and the magic users, that way the mundane don't need a gigantic army and just one that twice as large as the magic user army.
Also remember war isn't all about numbers and fire power, bad strategies, poorly trained soldiers and incompetent generals can destroy even the mightiest of empires