Is there any way for me to prevent sneaky countries (bloody Portugal) to declare war on someone I am warring and just go around and start sieging up everything? Kind of painfull when I am stuck in a battle with my main force just barely winning and noticing in the meantime that they went around and sieged up everything, making me unable to get the provinses, is there any way out of this except warring on Portugal?
Not really, the AI likes to jump on countries that are in trouble so if Portugal wants those regions and they have an oprotunity, they will take them. The only way you can really prevent this, short of going to war with Portugal, is to have your armies positioned to get to those regions first. If you were to tell us what region you're in we could probably help you more but if you could just get a few extra troops and drop them behind enemy lines right when the war starts (or after the first big battle begins). You'll get to those regions before Port can.