A pacifist is one who is opposed to the use of violence and war. What I plan to write is not a critique of their position. I am going to explain why I believe God permits the Christian to use force as a soldier or police officer in certain situations. I do not plan to deal with self defense or the use of force to defend others in a non-war setting, although I do believe that at times this is permissible.
War was allowed for Old Testament believers.
I think most would agree with me that in the Old Testament times, God allowed believers to go to war as these examples illustrate. Abraham, who lived before the Mosaic covenant, went off to war with his servants to rescue Lot and others who were taken captive by a group of kings. God included rules for warfare in the Mosaic covenant. See Deut. 20 Sometimes God called Israel to be His means of executing His judgement on a nation by attacking that nation or nations, like the Canaanites and Amalekites. Moses and Joshua led the people in battle as they brought God's judgment to the wicked nations of Canaan. But most of the time Israel went to war to defend themselves from various enemy nations. David was a great warrior, who fought many battles that were simply to defend Israel. Solomon, whose reign was one of peace, wrote that there is "a time for war and a time for peace." Ecc 3:8 War was part of reality for all the kings of Judah and Israel. Nehemiah and the rest of the builders armed themselves to defend Jerusalem which was being rebuilt. Throughout the OT, wars were fought by God's people. Nowhere do we find a prohibition for believers to not go to war or use force.
Both the Old and New Testaments call for believers to love their enemies.
People will say that in the New Testament God calls us to a higher standard of love and often point to passages like Matt 5 39-44, which say for example: "But do not resist an evil person; but whoever slaps you on your right cheek, turn the other to him also." and "But I say to you love your enemies and pray for those who persecute you." This passage is part of Jesus' sermon on the mount, in which Jesus contrasts the teaching of the rabbis of the day with His teaching. This is not a contrast between the OT and the NT. In fact nothing Jesus says in this sermon is really different from what was taught in the OT. The law of Moses taught that you should love your neighbor as yourself, which included your enemies. Proverbs 24:17 said, 'Do not rejoice when your enemy falls, and do not let your heart be glad when he stumbles." Proverbs 25:21-22 says, "If your enemy is hungry, give him food to eat; and if he is thirsty, give him water to drink; for you will heap burning coals on his head, and the LORD will reward you." This verse is used by Paul in Romans 12:20 to reinforce his instructions to love our enemies by doing good to them and leaving vengeance in the hands of God. Leaving vengeance in the hands of God was an OT teaching as well. Deut 31:35 says that "Vengeance is mine, I will repay." This verse is also quoted by Paul in this passage. I fail to see how there is any difference from the OT to the NT in regard to how we should treat our enemies. Both halves of scripture teach the same thing. So if going to war was allowed for believers in the OT, then why would it not be allowed in the NT?
The government is given the sword by God
God has given the government the authority and power to enforce the law of the land, even by lethal force. Romans 13:1 says "For there is no authority except from God, and those that exist have been instituted by God." Along with this position of authority God has given the government the power to punish those who disobey the laws. Sometimes this means using force to do so. The government has been given the sword to accomplish this as Romans 13:4 says, "But if you do wrong, be afraid, for he does not bear the sword in vain. For he is the servant of God, an avenger who carries God's wrath on the wrongdoer." The sword is a symbol of their authority, which gives them the right to punish those that break the law, even by capital punishment, that is the taking of someone's life. This agrees with the command by God in Genesis 9:6, where we are told that, "whoever sheds the blood of man, by man shall his blood be shed, for God made man in his own image." Killing someone is not always wrong, in fact it is wrong for a government not to kill those who murder another person. The government must kill those who have been thoroughly convicted of murder. Governments are allowed to use the sword to protect people from enemies inside the country and outside the country. This is the responsibility that God has given to them. When other countries attack, murder and pillage, then the government has the duty to protect its citizens from these external lawbreakers. This is God's call to those in authority, "Rescue the weak and needy, deliver them from the hand of the wicked." Psalm 82:4
Pacifism hinders the Divine task of the government
If we refuse to bear arms, we are hindering the government from doing its God given duty. Suppose if the majority of people in a country were Christians who refused to join the army when their country was attacked. How would the government be able to defend its people? Or what about enforcing the law inside the country with the police force, who may need to use lethal force to preserve the peace and uphold the law? Both these task are given to the government by God, so why should only unbelievers be allowed to help the government in their duty? No I believe it right for Christians to be police officers and soldiers. It is right because we are helping the God appointed authorities do their God given duties. It is also right because we show love to others by defending them against those who would seek to destroy them. Sometimes the only way to do that is by using lethal force since people are evil and often unreasonable.
Not all war is just
This does not mean that every war is right. Governments and countries often go to war for wrong reasons like conquest, revenge or wealth. Christians need to be wise about going to war. The Christian who with prayer and careful thought will most often understand the need of the hour. Sometimes that may be that we have to say no I can't join this war since it is not just. Then we need to accept the consequences knowing that we have done what was right before God.
War is a terrible and serious business. Killing anyone made in the image of God is a weighty act. An immortal soul is involved. As such we need to be very careful and wise before we take any action to take life. Taking life even when it is right and just should be done with sorrow and only when no other options present itself. But when it is our path of duty, we should do it with courage and conviction.
Lastly, we should long and pray for the return of Christ which will bring an end to war. When Christ shall reign upon the earth, "nation will not lift up sword against nation, and never again will they learn war." Isaiah 2:4b