One of the most iconic military uniforms ever is le pantalon rouge. The French military used navy jackets and red trousers from approximately 1829 to 1914 as a standard uniform for most regiments.
The British upper class also adopted the idea of red trousers. For that reason, they are considered snobbish by many, and that’s why most men rarely use them to this day – safe for American WASPs wearing red bermudas in the summer (which is also an iconic look that I personally like, but others seem to hate).
It’s a flashy combination, but stylistically it works. I would recommend, though, that you don’t wear red but rather burgundy trousers. Red is just too bright and will call for a lot of attention. Burgundy is more sophisticated and masculine. And it possibly goes even better with navy and other dark shades of blue.
Since blue and red are two primary colours, they should be separated by something neutral. Essentially, you should just wear a white shirt with this outfit. If that feels too formal or too boring, switch to a white shirt with blue stripes. In terms of neckwear, a burgundy tie, possibly striped and in combination with another colour, would work excellently.
For footwear you can do just about anything with this outfit. Dark burgundy tassel loafers, with leather in a slightly darker shade than the trousers, are a perfect match. Dark brown penny loafers will work as well. Black oxfords are another option – the French soldiers wore black shoes with their navy jackets and red trousers.