As someone who has lived his entire live in Porto (well, the suburbs), what exactly are you talking about? I'm guessing you went to the old part of town (Sé, D. Luís I bridge, Fontainhas/Ribeira)? More details would be nice, otherwise I'm just guessing here.

The only thing I can imagine a tourist seeing and going "shit's war-torn" would be the top-down view from the D. Luís I bridge into some of the abandoned houses that have caved in roofs and stuff.

Anyway, it's just a very, very old city, and although you can still see scars from old wars (the remains of the Fernandine wall for example), some parts are just dilapidated, simply the result of age, neglect and poverty. Not necessarily because of a recent war. Or the Estado Novo either, at least not directly.

Without having visited Porto, I suspect what you are seeing is just the standard population loss and that comes with de-industrialization. For much of the 20th century, Oporto was a regional center for northwestern Portugal, which was the main center of factories and heavy industry in Portugal. Like Sheffield, Lille, Detroit,and other Western cities that were highly dependent upon heavy industry, Oporto has probably seen a significant fraction of it's population and jobs leave over the last 30 years or so.

I think under that definition big chunks of Michigan, Pennsylvania and Ohio would look like they have gone through a recent war with large portions of Detroit and Cleveland being nearly abandoned.