before I conduct a war I check these things all out: Morale sliders, Leader Placement, set Slider to Mint if it's a serious war
Support limit for each province my troops with be stationed during their defensive, manuevering and attacks. The attrition levels of Enemy provinces
# Leaders cancel out attrition better. So do LandTechs... #
I check for Swamps, Mountains and Forests, they're Infantry Combat Terrain. Plains and Desert are <White and Yellow> Great for Cav. assaults. Forts give bonuses all around and must be taken into consideration. On defense, you need to rally troops in groups as you cannot Muster full Force in 1 Province to outmatch your enemy. Kind of like striking a man with a Jab, right hand, then an uppercut. Combinations!!! Are Key to Victory, but are costly if you don't get the hell out quickly as you'll amass attrition. You need anything over 40Warscore to Stab an enemy into defeat..
With EU2, Defense you may want to go offense if you have leaders and numbers depends on your style and force a end to the War. You can hold behind your lines but if you have a big front he may out maneuver you, you must be aware of what is a stabbing position for him. If he can take 2 of your CoTs and your Capitol you're toast and soon to be his Vassal. You lose Warscore rapidly if you lose Provinces and battles.
The key is this then, once you can win battles and take provinces, plus hold provinces and repell mass invasions both amphibious and over land. Is to know what will give him a Winning Score against you, what punches he'll pull and anticipate where he'll hit. It's usually quite obvious for each aggressor, especially a land one. Look for what benefits them most and fortify and prepare your troops/leaders/fleet/allies/whatever for fast manuevers to cover these areas OR better yet, go on the offensive and take from one of his Allies or from him an equal property or more forcing him into a defensive posture while on the offensive. This all has to be figured out by the MP, Leaders, Allies of your enemy nation... This is the basics... But you have to use them properly in EU2 War to win and most do not understand these concepts and lose. You can LOSE vast numbers of troops and win, you can be stabbed down to -3 and maxed out WE, 5 Loans out and still come out ahead. Though that's usually taking it too far, war is expensive, and if nothing else someone is bullying you, draw it out and make them pay for those petty provinces their stealing from you. Thatta way you get some payback!

The Cruel Way to give away property
On Defense you don't need nearly as many troops as unlike your enemy you can pull back and you can also attrit him<sometimes it's good to stack your men in your own Provinces where he incurs desertion and you do not, that's a whole different aspect to the game> If you see Winter comming, get out and let him dwindle, if you see him assualting and failing WAIT! Then counter on top of him. If you see a great leader, don't hit him, trail him... Follow his endtails cut him off and then when he's low on supply Burn his arse... Supply is traced through captured provinces in enemy territory and gets low when you don't have a adjacent territory conquored...
War is Blood! War is the Nature of the Beast! Your enemies are all around you!
Constantly be aware who is an enemy and who is a friend..they'll change constantly, you'll be a great warmonger and great player by mastering simple simple concepts.. it's a lot like boxing, block, block, jab, jab, jab, RIGHT HAND! NO! Fake out! You marched your troops right, but you stopped the last moment and marched the other direction

he stacked and wasted away!