Attack means you are trying to destroy something.
Defense means you are protecting something, trying to prevent something from being destroyed.
To protect something from being destroyed, you must destroy that which attacks it.
Thus, defense involves destruction.
However, the difference between attack and full defense is in their goals and priorities.
Lets say that my goal was to protect my country, while your goal was to destroy it.
My goal is to destroy attackers only in an amount which is enough to protect my country.
Thus, if I have 100 men, I put them all in defense.
Lets say that you also have 100 men.
You only have 2 choices: Put them all in attack or put some in attack and some in defense.
Since you are afraid of counter attack and also must protect your country, you cannot put all in attack.
So I can put 100 men in defense, but you cannot put 100 men in attack. You can only attack with less than 100 men.
My men are all together, but yours are separated on attackers and defenders. Since I dont attack your country, your defenders dont fight with my men.
So the number of my defenders outnumbers number of your attackers by default, thus the numbers on the battlefield favor me.
However, it doesnt stop there. Since you are attacker, it is you who will have to travel long distance to reach my country. Thus, you will have to use more energy, and will suffer more from traps/mines, guerrilla warfare, artillery, strikes...ect.
You may say that you have element of surprise (Hitler thought the same thing).
However, the problem with surprise is that it only works on countries who have no intelligence and observation services.
And surprise only works once. The day you attack, it will be very much clear that you are the attacker, and thats about as much advantage as you will get from surprise attack.
Further, any equal trade of loses favors me, because I have more men on battlefield.
Further, as long as I keep all my men together, you have no way of attacking isolated units because there are no isolated units on my side.
If you choose to spread your units everywhere so they are harder to stop, they will also be weak everywhere and easily flanked since they will be further divided. - in words of Sun Tzu.
However, if you choose to concentrate them, but my units are also concentrated, your units will be easy to find and easy to destroy and trap since my units outnumber yours.
So your only choice would be to spread your units, divide them on groups attacking different areas so that I cannot stop all groups.
However, my men who stick together will easily destroy those smaller groups one by one as soon as they encounter them.
Basically, your only hope is that all or most of your individual groups somehow dodge my one larger group, but since you will spread them over large area, it is not possible for them to dodge anything, really. If they are spread over large area, they will be present everywhere and my one group will meet them where ever it goes, and set traps for them easily.
Since I always have number advantage, irrelevant if you have one concentrated group or many smaller groups, you are always at a disadvantage, irrelevant of if your men attack all together or in group.
And this is even without the possibility of civilian population providing additional resistance to your men.
Basically, if you are attacker and I am defender, all else being equal, your men will travel longer distance and be in lower number, thus they have no realistic way of defeating my men.
The only thing you have working for you is element of surprise, which can even work against you later since if your men can hide somehow, so can mine.