Yes, there are major objections. I will only briefly touch on several. 
No doubt some of my readers have already scanned right past Romans 12 to chapter 13, a favorite proof text for justifying violence committed by “the governing authority,” (v. 1) which “does not bear the sword in vain” (v. 5). Nevermind that Paul is here explicitly addressing the duties of Roman Christians who are “subject to” (v. 1) these authorities–not the authorities themselves. This passage is for Christians who de facto do not carry the sword! Nor could it possibly be construed as justifying any violence beyond that of a state punishing its own criminal citizens, and even this could only apply insofar as the state is acting as “God’s servant for your good” (v. 6)–the same “good” which was just clearly defined in Romans 12, in terms of blessing persecutors and feeding enemies.
My primary response, however, is simply that before his conversion Paul was himself a state-instituted authority who wielded a sword, with terrible efficiency. Thinking deeply with a mind that had not yet been renewed and transformed, he completely believed this violence to be legitimate, righteous, and yes, God-ordained. But after he met Jesus Christ? Well, I’ll just let him speak for himself.
I was formerly a blasphemer, a persecutor, and a man of violence. But I received mercy because I had acted ignorantly in unbelief, and the grace of our Lord overflowed for me with the faith and love that are in Christ Jesus. The saying is sure and worthy of full acceptance, that Christ Jesus came into the world to save sinners—of whom I am the foremost. But for that very reason I received mercy, so that in me, as the foremost, Jesus Christ might display the utmost patience, making me an example to those who would come to believe in him for eternal life. — 1Tim 1:12-16
Paul explicitly connects his conversion to Christ with a total abandonment of coercion and violence–symptoms of ignorance and unbelief–in favor of grace, mercy, and a willingness to die for the Kingdom of God if necessary. Moreover, he takes this newfound refusal to pick up the sword as an example for all believers.  So, if you aren’t an agent of the state, Romans 13 in no way justifies any violence whatsoever. If you are, then it does not even justify defending your own country and its citizens from foreign attacks. And if you find yourself brandishing a weapon at someone as an agent of the state–dare I say it?–perhaps you should ask yourself where your deepest allegiance lies: an earthly kingdom or the Kingdom of God. 
“But!” you cry–I can almost hear it now–what about saving society from criminals, and children from murderers, and Europe from Hitler, and America from the Terrorists? In sum, what about all the innocent people whom we have a duty to protect from the evildoers? 
The New Testament itself is uncomfortably silent on this topic; while we are absolutely called to “care for the orphans and widows in their distress,” (Jm 1:27) there is no explicit command to believers regarding the use of “selfless” violence (except of course Jesus’ emphatic condemnation of violence on his own behalf; more below). I think this fact speaks for itself, as does the martyred witness of the rapidly-growing early Church  before “the accession of Constantine terminated the pacifist period of church history.” 
Ever since Jesus’ name was first attached to political and military power claims around 300 C.E., the Church has traditionally turned to many great and sincere Christian thinkers–from Origen to Augustine, from Calvin and Luther to C.S. Lewis–who have rejected nonviolence in favor of various forms of “just war” theory.  Some of their cases are quite compelling  and there are certainly a number of philosophically sound arguments to be made for protecting innocent people with violence on an individual and corporate basis. I want badly to embrace such arguments, because I too feel the philosophical and emotional weight of them. Many more great and sincere Christians have taken these writings to heart as justification for all sorts of violence perpetuated by themselves and others, even going so far as to label them “doctrine.”
Notice some common threads running through all of these arguments. Each of them is constructed from premises which are intuitively obvious–eg., people like Hitler must be stopped by any means necessary; defending ourselves and our loved ones with violence is justified; it can be loving to kill an evil person; the right intention justifies a violent action; a child is more valuable than a murderer; violence is a successful solution; if we don’t act with force then no one (including God) will help; God favors His chosen people over and against their foes–and then works backward from these premises to reinterpret the NT nonviolence teachings, or ignores them altogether. Each of them calls us to act primarily out of fear (however healthy or apt), rather than out of love. Each of them leaves out the premise that the God of Jesus Christ is really in control, and really knows what is best. Obviously Jesus and Paul could not really have meant that we should die praying for the nasty, evil, terrorist kind of enemies rather than retaliate; then inevitably we would all be slaughtered and the enemy wins! So, they must have been talking about only certain kinds of “enemies,” or using “love” as more of a metaphor or feeling of the heart, or sketching an abstract idealization, or subliminally limiting their instruction to the individual lay-citizen, but not the state.
The only problem is the blatant lack of New Testament foundation for any of these “obvious” premises; they simply do not appear. Actually, the ultimate “just war” would have been to defend Jesus with violence, but Jesus vehemently rebukes Peter for attempting exactly this: “All who take the sword will perish by the sword” (Mt 26:51-53, cf. Lk 22:49-51). Jesus then heals his aggressor before informing Pilate that his followers’ nonviolence is the proof that his Kingdom is supernatural (Jn 18:36).
Nor do I find these non-pacifist reinterpretations of Scripture tenable. Probably the most pressing question is, “Who then is my enemy?”  Jesus and Paul were clearly talking about extremely real, extremely evil enemies: the ancient Romans who dominated and terrorized their society, brutally murdering both of them and most of the other early Christians. What makes our enemies, our enemies, is the fact that they threaten to harm and/or kill us or our loved ones. (Unless it is supposed that it is merely the fact that they don’t like us, or that we don’t like them, or that we disagree with them about some issue, or that we pose a threat to them, none of which provides any possible justification for doing violence to them.) But this is also the very fact which is used to try to justify violence against them. In the cases of “selfless killing” we would merely expand the category of “our loved ones” to whatever ‘innocent’ people are being threatened, eg., an ethnic minority. And certainly as Christians we ought to consider all (‘innocent’) people our loved ones in some sense! Thus, a psychotic murderer is absolutely my enemy insofar as he poses a threat to me, my loved ones, or any ‘innocent’ person. Criminal leaders (eg., Hitler, Osama bin Laden) and their followers could also be generalized as “enemies of the state” insofar as they pose a safety hazard to society at large. What is clear here is that whether we are concerned to protect a people group or an individual, we could only possibly justify doing violence to the aggressor because he is an aggressor. But–here’s the rub–this automatically places him precisely in the category of “enemy” whom Jesus and Paul commanded us to love, pray for, and bless! Moreover, all of the abundant New Testament commandments to love are extremely practical and action-oriented. There is not even a hint of love as merely an intention, feeling or concept; such an attitude is explicitly condemned!  Jesus repeatedly connects the practice of enemy-love to being a child of God, so the commandment applies to all who wish to be His children. And nowhere do we find any qualification such as, “unless you’re a soldier or a government official.”
So what if instead, we started with the New Testament itself as our main premise, and then worked out our arguments? I think we’d get some rather different results. I’m not saying there are any easy answers to these enormously difficult cases. I have no solutions on hand for the government official who wants to prevent genocide, or the parent of a kidnapped child. I’m just pointing out that these questions are extreme examples, and that Jesus doesn’t even try to answer them. Rather, he calls each and every one of his followers to die to our need for self-defense and to refuse to rationalize the violence that feels so righteous to us. We are to instead cultivate the sort of character which will instinctively seek nonviolent solutions, which will react out of love rather than out of fear (cf. 1 Jn 4:18), which will lay down one’s life rather than take a life. He calls us to put the message of His cross–which is foolishness to the world, but is the power of God to those who are being saved (1 Cor 1:18)–back into the center of the calculus, where it belongs.
Importantly, we are not to read these nonviolence teachings as some sort of quick-and-dirty instruction manual to consult when we find ourselves facing particular instances of violence against ourselves and others. Rather, as with most Biblical wisdom teaching, the practice and pursuit of nonviolence is a discipline which changes and develops our character in a much broader and deeper sense. Hence the call to pray for our enemies: this is not a kind of emergency tool to deploy just when we feel threatened or angry. It is a daily practice which will grow us into the kind of people who can wisely respond to situations when our “enemies” actually confront us. For example, if my character is shaped so that when I hear a man breaking into my house, my first thought is “This is a precious child of God whom I am called to love like Jesus, who died for him,” rather than “Oh no! Protect me and mine! With violence if necessary!” then I will have a much better chance of finding a creative, nonviolent solution in that instance. Moreover, if I have already forgiven him, then I will not seek violent retribution. But a nonviolent reaction is not nearly so realistic if I wait until that moment to ask myself the “WWJD” question.
Clarity often comes only with obedience. So, perhaps if we start with Jesus’ teaching as our presupposition instead of generating our own, if we begin to align our hearts and lives with his commandments, if we cultivate a cruciform, self-sacrificial lifestyle–then as we do so, this foolish and insane teaching might start to make sense to us. Perhaps if we can rebel against our fallen nature by submitting our ethics, philosophy and politics to Christ, we will start to see the bigger picture and the supernatural logic of His Kingdom. Perhaps if we truly die to ourselves, He might just resurrect us.
 To wit, I will not address the issue of reconciling NT non-violence teaching with various OT passages where God appears to command genocidal violence and other atrocities, except to note that Jesus is the authoritative revelation of God as compared to the Old Testament: Cf. Heb 1:1-3; Jn 5:36-47, Lk 24:25-27 (all scripture testifies about Jesus); Jn 14:7-9 (“Whoever has seen me has seen the Father”); Col 2:9, 1:19. Furthermore, Jesus repeatedly and explicitly subverts OT violence practices, eg. Mt 5:38-48 (“You have heard it said…But I say to you”) and Lk 9:54-56 (contra Elijah’s righteous conflagration). For an exhaustive pacifist discussion of the apparent OT-NT contradiction/discrepancy, see Greg Boyd’s forthcoming Crucifixion of the Warrior God and the multi-authored A Faith Not Worth Fighting For (ed. Tripp York, Cascade Books, 2012).
 Nor will I attempt to address the book of Revelation; many scholars have argued successfully that it is actually a brilliant subversively manual on nonviolent spiritual warfare. See, eg., R. Bauckham, The Theology of the Book of Revelation (Cambridge University Press, 1993), S.K. Tonstad, Saving God’s Reputation: The Theological Function of Pistis Iesou in the Cosmic Narrative of Revelation (T&T Clark, 2006), L. Johns, The Lamb Christology of the Apocalypse of John (Mohr Seibacl, 2003). Boyd’s article “Revelation and the Violent ‘Prize Fighting’ Jesus” (available at http://reknew.org/2010/09/revelation-and-the-violent-prize-fighting-jesus/) succinctly outlines this position.
For a more thorough (and fascinating) treatment of Paul’s personal and theological non-violent paradigm, see Derek Flood’s “The Way of Peace and Grace,” available at http://sojo.net/magazine/2012/01/way-peace-and-grace
 It is often objected that the NT includes several clear approbations of soldiers (centurions) which do not include any condemnation–or anything beyond passing mention–of their occupation: Mt 8:5-13 (“nowhere in Israel have I found such faith”), cf. Lk 7:1-10; Acts 10:1-2 (“He and all his family were devout and God-fearing”). Notice that these men’s total submission to Jesus Christ’s authority (“Lord, I am not worthy to have you come under my roof”) and their counter-cultural use of their own authority to bring their households under his Lordship is what marks them out as faithful.
 See J.J. Porter’s fantastic Ichthus article “Must Christians be Pacifists?” (Vol 5, #1, 11/20/09) for a thoughtful overall treatment and heavy critique of pacifism on this line of objection. As Porter argues, there is little distinguishable moral difference between individual “selfless” killing and large-scale “just wars” so it is fair to address all such cases of “protecting the innocent” under the same heading.
 See Porter’s article for a nice collection of nonviolence quotes from the earliest church fathers.
 Roland Bainton, quoted in Robin Gill, A Textbook of Christian Ethics, 3rd ed (Continuum, 2006), p. 194.
 See David G. Hunter, “A Decade of Research on Early Christians and Military Service,”Religious Studies Review 18.2 (April 1992), 87–94; and Louis J. Swift,The Early Fathers on War and Military Service (Wilmington, Delaware: Michael Glazier, 1983).
 Though, alas, Lewis’ “Why I am not A Pacifist” is easily his sloppiest work that I’ve read.
 Cf. Lk 10:25-37, the parable of the Good Samaritan: “Who then is my neighbor?” Note that Samaritans were natural enemies to Jews in this context, and that he who actively demonstrated mercy to his natural enemy was his true ‘neighbor;’ moreover, we are commanded to “go and do likewise.” (v. 37)
 Cf. Rom 12; the book of James esp. 1:22, 2:14-26 (“Faith without works is dead”); 1 Jn 3:14-19, etc.