Answer:
Resources, Man Power, Technology, Politics, Social & Economic Structures, Population to a certain degree, Military Experienced leaders, but most importantly, the Will Power to Fight.
Explanation:
Of course, there are many other different factors, but these are some of the more major ones. To break it down to you why these are on the list:
Resources: Resources are needed for anything, not just for war. But in the case of war, resources are needed more than ever. In many cases, certain resources & raw material (such as steel, certain types of food, wool, etc.) are limited to the general public as they are used to help equip the common soldier to be able to better their survival out in battle. Resources such as iron & steel are also used to make weapons of war to give the edge to one side or the other.
Man Power: At least until the recent hundred years, the side that had more better equipped soldiers, as well as having a superior man power generally won. Those who were able to beat more with less (for example: Battle of Muye in 1046 BC, Battle of Marathon in 490 BC), are hailed as heroes, and their military leaders are studied. However, as a rule of thumb, more is better.
Population: Without a large population, you cannot call up a larger army. As stated in Man Power, more soldiers (that are equipped to a degree, and have the training), typically means a higher chance in winning. Without the population to back it up, you cannot raise large armies.
Military Experienced leaders: One of the key proponents of winning a war. The leader, whether political or military, must have some sort of military experience to understand what is going on. In the form of political leaders, typically they would be a set of advisors, while a military leader would have the experience necessary on demand to make changes to plans or act on them.
Will Power to Fight: This is especially true in the United States. Our leadership entirely depends on the support of the people. For example, wars that impact us directly (World War I, World War II, for example), seem to see a larger population support and a willingness to fight. Wars that seem to not have a negative support (Vietnam War, for example, where the US was winning, but had to withdraw because of protests by the American public), led to us losing. Of course, this can apply to almost all nation, but the US is a prime example.
Of course, remember, there are many other reasons, both large and small, that determines an outcome of a war. However, these are some of the more major ones.
