How would a country reply to a declaration of war?

Once war is declared, it's war. End of. The only possible response is military (not milatery).

A country wouldn't have declared war unless

a) its intention is to invade and conquer, so it's not interested in doing anything diplomatic, or

b) it had tried everything else first to avoid war, and come to the conclusion there is no alternative.

So trying diplomacy makes no sense. If that was going to work, it would have already been done.

Consider how WWII started. Germany had been invading other countries for the previous three years, nobody tried to stop him, and finally the British Prime Minister, Neville Chamberlain, realised that just talking to Hitler was doing no good at all. Britain had made a promise to Poland that it would help if Poland were attacked. Even when Germany DID invade Poland, Britain went for "one last chance" - not that anyone really expected Hitler would take any notice, but at least Britain could say it had done the right thing. It issued an ultimatum to Germany, "get out of Poland by noon tomorrow or we are at war". Hitler ignored that, therefore it was war.

Which rather stunned Hitler - he'd been so used to lying to Chamberlain and expecting him to do nothing. Especially after meeting him in 1938 and Chamberlain came home waving his "piece of paper" signed by Hitler that said Hitler "had no more territorial ambitions in Europe". What a big fat whopper of a lie if ever there was one. And when Germany invaded Poland, Chamberlain looked such a fool for believing it.

See - Britain declared war because it didn't want war, and had already tried whatever it could to avoid it. A formal declaration indicates that a country is trying to be nice, but now it's the only thing left it can do. If you want to invade another country, you don't declare war, you just DO it - why would you want to give that country a chance to prepare a response? As Germany did with Poland - it didn't declare war, it just invaded.

So why would Portugal declare war on Spain? If that's what it wanted, it would just attack Spain. An actual declaration of war is very rare and only ever happens to join in a war that is already going on.

If Portugal declared war on Spain you could be sure the EU would drag them both into committee and tell them to knock it off.

I would say Spain would have to respond with military force either by going on the defensive or to go on the offensive.

HI sounds like you know absolutely nothing of diplomacy and why would a small country declare war on its larger and more powerful country neighbour. Only a right bunch of idiots would consider it unless they are really stupid.