Justifications for War:

In the Distant Past:
1) Defending the Empire.
2) Expanding the Empire.
3) Honoring an alliance.

In the near Past:
1) Expanding the Empire.
2) Honoring an alliance.
3) Defending the economy of one's nation.
4) Creating a better union (er, democracy for some, while still promoting the slavery of others).

In the recent past (for world powers only):
1) Expanding the economic expansion of their corporations.
2) Responding to nations intent on expanding theirs (corporate expansions).
3) The inevitabilty of someone more evil than those ruling one's nation that justifies their intervention on another.

The Present: [note: for a dictatorship no reason has ever been required to proclaim war on another nation - they simply had to have their propagandists come up with a cause for the peasants to die for. For those proclaiming democracy; they must atleast demonstrate that a war is for the common good of their citizens that their sons are dying with a purpose or the purpose to defend them (that or give them enough money to make them shut the f' up - it was $5K in WWII, $10K in 'nam, and $10K in Iraq up 'til a few months ago - when it was raised to $100K), or come up with a neet catch phrase - like, "Remember,.....what ever". Works every time.]
1) WMD (defending the empire)
2) Spreading of Democracy
3) Oil (the need to defend the economic interests)
4) expansion of the Empire
5) My nation right or wrong
6) ignorance really is bliss; if one accepts the loss of a son, husband, or father as being justified for the false premises, lies, and propaganda tossed at them by accepting the idea that "we must not let them to have died in vain" (versus having died for big oil, or some egotistical president and his staff that never, ever heard the shot fired in anger). Dying in vain is what soldiers do. Dying in vain is why soldiers are physically trained, emotionally created, taught to supress their convictions and learned to kill on impulse. It is also why soldiers love one another, and are so unwilling to accept that one of their brothers was allowed to die without a cause - for something as inmaterial to them as oil. Oil is not a cause, yet it is. To some. For a soldier? It is simply about defending a brother. Dying for a fellow soldier, so that he might live or simply responding to training (BS) - it is about the man beside you. It ain't about a country, so much as it is about just that - the men next to you.

As for nations?
Reasons to go to war? Today: Oil, water, food, the propagation of the specifies, religion, the expansion of the empire. Samo=samo. The only things that have changed are the means with which to kill, not the ideas of those that propagate their reasons for it. Those with what they preceive as ultimate power (especially the weak thinkers) always believe they bully the weak - especially when they do not understand them. It has always been, and may always be. The unfortunate thing is, now we got like 10 nations with nuclear capabilty and all have reasons to go to war.

The more things change, the more they remain the same.