The Holy verse says:
"And let not those who disbelieve think they will escape. Indeed, they will not cause failure [to Allah]. And prepare against them whatever you are able of power and of steeds of war by which you may terrify the enemy of Allah and your enemy and others besides them whom you do not know [but] whom Allah knows. And whatever you spend in the cause of Allah will be fully repaid to you, and you will not be wronged." (8:59-60)
Who is the verse (8:59) talking about?
This verse was revealed in reference to the Battle of Badr which took place between the Muslims who sought refuge in Madinah and the Quraysh who had persecuted them and driven them out of their homes in Makkah. In the battle of Badr the Muslims won.
What the disbelievers did was explained in earlier verses of the same chapter (surah) like in the verse. The verses says: "And remember when you were few and oppressed in the land, fearing that people might abduct you, but He sheltered you, supported you with His victory, and provided you with good things – that you might be grateful." (8:26) and the verse "And [remember, O Muhammad], when those who disbelieved plotted against you to restrain you or kill you or evict you [from Makkah]. But they plan, and Allah plans. And Allah is the best of planners." (8:30)
and the verse "Indeed, those who disbelieve spend their wealth to avert [people] from the way of Allah. So they will spend it; then it will be for them a [source of] regret; then they will be overcome. And those who have disbelieved – unto Hell they will be gathered." (8:36)
So the disbelievers fought the Muslims, prosecuted them, boycotted them, took their money and properties, tortured them till they left no choice for the Muslims but to escape to Medinah leaving their home-land. The battle of Badr was the first official battle between the Muslims and the disbelievers of Quraish, their earlier aggressors. After the battle some of the disbelievers were killed, some held captives and some ran away.
Who is the verse (8:60) talking about?
The noble verse is telling Muslims to be prepared against THEM. This "THEM" refers to
1- The disbelievers who fought the Muslims
2- The traitors who were mentioned in an earlier verse "If you [have reason to] fear from a people betrayal, throw [their treaty] back to them, [putting you] on equal terms. Indeed, Allah does not like traitors." (8:58). In Islam if you make a treaty with someone and that person betrays you, you have to notify him first that you cancel the treaty then fight him.
3- Enemies the Muslims know nothing about, but God knows their intentions very well, who are planning or even considering attacking Muslims for whatever reasons.
Is the verse (8:60) REALLY calling for war?
Look carefully at the word "terrify" so the whole idea about getting prepared for the enemy is not to actually "FIGHT" or "KILL" but to "Terrify" the enemy which means that you get prepared by your maximum capability to reach a military power that will make your enemy think twice before attacking you. Isn't this like the military parade that all countries now perform to tell the world "We are strong...think twice before attacking us".
Actually this is a cause of peaceful balance in the world because if a country is weak, then the stronger countries will never hesitate to try and solve their economic problems on their expense and hence wage war for any fabricated reason just to put their hands on the weaker country resources. So being prepared is a major cause for a country to live in peace without threats and hence protect itself and its' inhabitants.
The verse has no meaning of transgression in any way, it is not ordering Muslims to be prepared to "ATTACK" or "Kill" but for the sole reason of making the enemy exclude any wicked idea to attack them.
The proof of this is in the verse immediately after it. The verse says: " And if they incline to peace, then incline to it [also] and rely upon Allah. Indeed, it is He who is the Hearing, the Knowing." (8:61)