The term "pacifist" means many things to many people, as does "nonresistant". I know Mennonites who have no problem whatsoever with war, supporting war, or supporting those who advocate and wage war, all the while claiming they are "nonresistant". They just won't personally participate in war. They back this with what I consider to be a perversion of Romans 13.
If "nonresistance" is "... yielding to the circumstances that God has placed you into", why not yield to the draft (when the US had compulsory military service)? Interestingly, that was Martin Luther's approach in his "Two Kingdom" theology.
While today all too many Brethren are falling away from the teachings of Jesus, The Church of the Brethren historically has taken a unique approach among the various denominations that call themselves "Christian" that is true to Christ's teachings in the Sermon on the Mount (Luke 6 sand Matthew 5-7). While for many decades the Church of the Brethren has formally declared that "All war is sin", their understanding that all war is sin goes back to their founding in 1708.
But the Brethren statement that "All war is sin" is not meant to imply that God sinned in the Old Testament when he called the Israelis to war. While there are those in recent history (such as George W. Bush) who have claimed that God called them to wage war, it is important to note the difference between how God led the Israelis into war in the Old Testament, ensuring the Israelis went to war against overwhelming odds so that it was clear to all that it was God who was fighting for them, and the modern day George W. Bush "shock and awe" approach to war which uses overwhelming force against a comparatively defenseless "enemy". While a "David against Goliath" style war does not necessarily ensure that God is on David's side, the Old Testament makes it clear that when God did use his people to wage war he made it clear that it was God who was doing the fighting by victories that required miracles like the collapse of the walls of Jericho and the flight of tens of thousands of soldiers when a few men blew their trumpets. But this was all Old Testament.
Christ has given a new commandment, not only to "love one another", but also to love and do good to our enemies (John 13:34, Luke 6:27-36, Matthew 5:38-48, Romans 12:9-21, 1 John 2).
The polemics between the teachings of Jesus and the governments of this world are made clear by the definition of treason in the US Constitution of "giving aid and comfort" to the enemy (Article III Section 3), while Jesus commands us:
"Love your enemies, do good to those who hate you, bless those who curse you, pray for those who mistreat you. If someone slaps you on one cheek, turn to them the other also. If someone takes your coat, do not withhold your shirt from them. Give to everyone who asks you, and if anyone takes what belongs to you, do not demand it back. Do to others as you would have them do to you.
“If you love those who love you, what credit is that to you? Even sinners love those who love them. And if you do good to those who are good to you, what credit is that to you? Even sinners do that. And if you lend to those from whom you expect repayment, what credit is that to you? Even sinners lend to sinners, expecting to be repaid in full. But love your enemies, do good to them, and lend to them without expecting to get anything back. Then your reward will be great, and you will be children of the Most High, because he is kind to the ungrateful and wicked. Be merciful, just as your Father is merciful." -- Luke 6:27-36
And Jesus makes clear he means what he says by also telling us in John 14:23-24, “Anyone who loves me will obey my teaching. My Father will love them, and we will come to them and make our home with them. Anyone who does not love me will not obey my teaching. These words you hear are not my own; they belong to the Father who sent me."
And Jesus by his words in Matthew 7:21-23 also makes it clear that there will be many imposters who refuse to do what Jesus commands while claiming to be Christian:
“Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but only the one who does the will of my Father who is in heaven. Many will say to me on that day, ‘Lord, Lord, did we not prophesy in your name and in your name drive out demons and in your name perform many miracles?’ Then I will tell them plainly, ‘I never knew you. Away from me, you evildoers!’"
See also Matthew 7:24-27 (the parable of building your house on sand versus rock) and Luke 10:25-37 (the parable of the Good Samaritan).
But neither are true Christians pacifists. They are indeed warriors for God, but they do not fight with the weapons of this world, but instead use the spiritual weapons of God.
Jesus said, “My kingdom is not of this world. If it were, my servants would fight to prevent my arrest by the Jewish leaders. But now my kingdom is from another place.” -- John 18:36
And:
"Finally, be strong in the Lord and in his mighty power. Put on the full armor of God, so that you can take your stand against the devil’s schemes. For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms. Therefore put on the full armor of God, so that when the day of evil comes, you may be able to stand your ground, and after you have done everything, to stand. Stand firm then, with the belt of truth buckled around your waist, with the breastplate of righteousness in place, and with your feet fitted with the readiness that comes from the gospel of peace. In addition to all this, take up the shield of faith, with which you can extinguish all the flaming arrows of the evil one. Take the helmet of salvation and the sword of the Spirit, which is the word of God.
"And pray in the Spirit on all occasions with all kinds of prayers and requests. With this in mind, be alert and always keep on praying for all the Lord’s people." -- Ephesians 6:10-18
See here for an online readable version of
The Church of the Brethren and War, 1708-1941 by Rufus D. Bowman:
Full text of "The Church of the Brethren and war, 1708-1941"