Well zero is the first game in the storyline canonically, so not that hard to follow story wise. It's why it's called zero. You could play ace combat 5 first before zero, and stuff will make more sense that way as zero is a prequel released after 5, but doing so will spoil part of the ending of zero.
So it's a trade off you make, if you want to understand everything clearly and play in release order, 5 then zero, if you want the plot to unfold in canonical order and get surprised by all the plot twists, zero then 5.
Well zero is the first game in the storyline canonically, so not that hard to follow story wise. It's why it's called zero. You could play ace combat 5 first before zero, and stuff will make more sense that way as zero is a prequel released after 5, but doing so will spoil part of the ending of zero.
So it's a trade off you make, if you want to understand everything clearly and play in release order, 5 then zero, if you want the plot to unfold in canonical order and get surprised by all the plot twists, zero then 5.