What does Buddha and Buddhism say about fighting against enemy of dharma? What does Buddha and Buddhism say about fighting against an enemy who is hell bent on wiping out all buddhists in the name of religion? Should a buddhist actively participate in war against evil? I would like to have an answer both from a buddhist layman and a soldiers perspective. An active resistance to injustice in general. Thank you.
I personally find it difficult to accept that Buddhism is compatible with violence:


The first precept is I undertake the training rule to abstain from killing.
On the eightfold path, the definition of right and wrong livelihood (career) prohibits Business in weapons (also, slavery and prostitution, meat, intoxicants, and poison)
When you consider whether you should be "fighting against an enemy who is hell-bent", remember the verses at the very beginning of the Dhammapada,


  
  Mind precedes all mental states. Mind is their chief; they are all mind-wrought. If with an impure mind a person speaks or acts suffering
  follows him like the wheel that follows the foot of the ox.
  Mind precedes all mental states. Mind is their chief; they are all mind-wrought. If with a pure mind a person speaks or acts happiness
  follows him like his never-departing shadow.
  "He abused me, he struck me, he overpowered me, he robbed me." Those who harbor such thoughts do not still their hatred.
  "He abused me, he struck me, he overpowered me, he robbed me." Those who do not harbor such thoughts still their hatred.
  Hatred is never appeased by hatred in this world. By non-hatred alone is hatred appeased. This is a law eternal.
  There are those who do not realize that one day we all must die. But those who do realize this settle their quarrels.
  



However, there are nominally Buddhist countries, which have soldiers. For example,
In Defense of Dharma: Just-War Ideology in Buddhist Sri Lanka includes innumerable quotes, from people who are arguing for and against violence, including quotes like,



  "Though some Sri Lankan Buddhist interpreters of Buddhism, including the
  Venerable Walpola Rahula, have claimed that “violence in any form, under any
  pretext whatsoever, is absolutely against the teaching of the Buddha,” and thus
  of Buddhism, some Buddhists that I interviewed in Sri Lanka during the summers
  of 1997 and 1998 suggested that war can be justified if certain criteria are met. In
  this chapter, I shall highlight the “just-war thinking” that a section of Sinhala-Buddhist Sri Lanka argues shaped Dutugemunu’s conscience, and examine what
  foundation there is, if any, for such thinking in Theravada Buddhism, in general."


  "we have to peacefully fight our enemies"


  "killing to defend the
  country or the sasana, therefore, brings few negative karmic repercussions, because
  the intention is to save, rather than to harm"


  "As we determined in Chapter 2 of this study, due to monks’ perceptions of
  themselves as “sons of the soil,” or bhumiputras, the sangha perpetuates and
  reinforces the idea that Sri Lanka belongs to the Sinhalas who are Buddhist. As
  Professor Aryarasinghe’s political party’s platform makes clear, however,
  bhumiputra ideology is not the monks’ alone; lay Buddhists, too, deem themselves
  to be sons of the soil. As we have also seen, the commitment to the soil might
  allow for justifications of violence, even war, if certain conditions are met. At the
  same time, there are a number of lay people and, in particular, monks, who are
  committed to their other “historic” mission, that is, to being exemplars of the
  Buddha’s message of peace, to being sons of the Buddha, or Buddhaputras."


  
    "Each of us should contribute to the cooling down
    process. If one individual is peaceful it means one less in the violent
    crowd."
  
  
  In my interview with Premaratne in 1998, he steadfastly argued that the civil
  war will not be solved in the battlefield; instead, according to Premaratne, we can
  find a resolution to Sri Lanka’s problems in the paradigm of the Buddha’s life and
  in Buddhist ideas about the self. In Premaratne’s view, Buddhism never justifies 
  war, and the fact that the Buddha chose to be a universal teacher (world renouncer)
  rather than a cakkavati (world conqueror) demonstrates that peace is superior to
  war."



Specific, notable Buddhists including the Dalai Lama seem to say that it's sometimes "necessary".

So in summary it seems to be difficult to be completely clear about what "Buddhism says": because it's possible to find "mixed messages" on this subject.

I personally think that:


"Abstain from killing" as a number one precept is pretty clear. It reminds me of this article, published in a satirical newspaper (soon after September 11, 2001): God Angrily Clarifies 'Don't Kill' Rule.
Saying that "killing terrorists doesn't count as killing, they deserve it, they brought death upon themselves, they made me do it" seems to me very dubious logic. For example, quoting from the Mahavamsa as written in the "Just-War Ideology in Buddhist Sri Lanka",


  Dutugemunu asks them
  how he will ever find comfort, considering what he had done, that he had killed
  such a lot of people. The arahants respond with their own just-war thinking:
  
  
    Only one and a half human beings have been slain here by thee, O lord
    of men. The one had come unto the (three) refuges, the other had taken
    unto himself the five precepts. Unbelievers and men of evil life were the
    rest, not more to be esteemed than beasts. But as for thee, thou wilt bring
    glory to the doctrine of the Buddha in manifold ways; therefore cast
    away care from the heart, O ruler of men.
  


Unfortunately this reminds me of the Nazi arguments/propaganda used to commit genocide in Nazi Germany: that people such as the Jews were "sub-human" and, also, were "a threat" (a threat to God's order; threat to German society; threat to German morality; threatening to enslave hard-working Germans; a deadly threat not only to the survival of the German people but to the survival of the world!)
I also don't see how Christians justify war: "Turning the other cheek" seems to me unambiguous. I know that people who are nominally Christian have tried to justify it with the theory of just war but that doesn't seem to me to be Christian: maybe it's life (and death), maybe it's the world, but it's not Christian, IMO.
Supposedly Buddhist doctrine such as,


  "The Buddha replied, 'He who deserves punishment must be punished. And he who is worthy of favor must be favored. Do not do injury to any living being but be just, filled with love and kindness.' These injunctions are not contradictory because the person who is punished for his crimes will suffer his injury not through the ill-will of the judge but through the evil act itself. His own acts have brought upon him the injury that the executors of the law inflict. When a magistrate punishes, he must not harbor hatred in his heart. When a murderer is put to death, he should realize that his punishment is the result of his own act. With his understanding, he will no longer lament his fate but can console his mind. And the Blessed One continued, 'The Buddha teaches that all warfare in which man tries to slay his brothers is lamentable. But he does not teach that those who are involved in war to maintain peace and order, after having exhausted all means to avoid conflict, are blameworthy."


... seems to me to come from a different tradition (a non-Buddhist Dharma), i.e. it's the central message of the Bhagavad Gita, that man-the-killer is an instrument of God ("Facing the duty to kill his relatives, Arjuna is counselled by Krishna to fulfill his Kshatriya (warrior) duty as a warrior and kill.") ... or in the quote above, a mere instrument of karma.


There's a saying, in the west, that "war is hell".

There is, similarly, quite a famous Zen story on that subject.

As for whether you should "actively participate in a war against evil", or how to participate if you must, my knowledge of non-Buddhist history suggests there have been a (relatively very few) people in the west who, calling themselves "pacifists" and "conscientious objectors", served in non-aggressive, altruistic, personally dangerous roles: for example disposing of unexploded bombs, or acting as medics, and as front-line stretcher-bearers (rescuing the wounded).