While the cons of war are myriad, including loss of life (both military and civilian), extreme economic strain and political or social disputes at home, there are nevertheless times when war may be said to be justified, such as when it functions as a measure of national self-defense or else as a means to prevent or alleviate a human rights violation. It is not always clear when wars are fought for a "just cause," however.
Often, a war that starts out as "just" will lose public support fairly rapidly after it has begun. Such was the case of the Bush administration's 2003 war in Iraq. Although around three quarters of the American public supported the military action when it commenced, only half as many were in favor of it after three years.
Wars of punishment may be accepted as just by some, but not by everyone.
There are times when war is inevitable, such as when forced to defend against an enemy invasion. In such cases, most would agree that war is justified. However, wars might be argued as self-defense on more dubious grounds as well, such as the assassination of a leader, an attack on an overseas embassy, an attack on an ally or the imposition of a trade embargo. Even something as non-aggressive as flag-burning might be viewed as "just cause" for war.