Why did Jesus tell His disciples to buy a sword?

I don't think the two acts are related I read it just as it is. Jesus showed them the importance of a sword to defend themselves and others but when Jesus was arrested he told peter not to fight for this is my fathers cup and it must be fulfilled. If Jesus had a army of 1000000 that undoubtly could defeat Rome would he to not call them off because it was him going about his fathers will? To say that he uses swords for one event only and the cutting of the ear to rebuke violence is adding context that is not there to me.

At the end of the day, the New Testament is clear to turn the other cheek. Yet, as we live through the Tribulation, it is impossible for me to simply stand back and watch the slaughter of innocence. Can we be expected to merely stand back as our children are beheaded? Do we lack faith if we fight back? God has given us an inherent instinct to protect life. To feed the hungry and put down evil, not to give into it and throw yourself into the fire. Even animals will run or fight to protect the sanctity of life. Knowing we will die is a fact. Whether we fight or stand still we all will die in our bodies here. The difference is that the strong and faithful in Christ, will be with God, in spite of physical death regatdless. So, I personally believe that we must literally pick up the sword and use it to defend.

For me, and I can speak for no one else, the answer to what Jesus said about arming one's self is found in Romans Chapter 12: "Insofar as possible, live at peace with all men."
The key phrase is "insofar as possible." While no one despises war more than those who have been a part of it, I know that violence is to be avoided but I'm not anxious to be a martyr without making a few martyrs of those who would take my life, and I refuse to stand idly by while my loved ones are martyred. So, what I believe Jesus was telling us is that we sometimes are forced to make a choice to either die or survive, and if I should happen to forfeit my eternity in Heaven because I chose to defend myself and others, then that is for me to decide.

Why did Jesus tell His disciples to buy a sword?

When Jesus offered the kingdom to the nation of Israel it was a legitimate offer. He did not wink at the disciples and say, "I know you guys are going to blow it, so here is the real plan".

The words of Jesus, even the prophetic words are true regardless of what Israel does.

If Israel had accepted Jesus anytime up to this point, the disciples would be rulers in Israel.

Now that Jesus is about to be put to death, there is a transition from the supernatural protection and provision of the disciples to a more natural existence.

The sword at the time is representative less of combat as we are likely to think today than it was of a normal tool used in traveling such as for processing kindling and firewood as well as protection.

The sword is rather emblematic of a hiatus in the unfolding of the kingdom.

It was a plot device.

A couple of scenes later (vs 49-51) shows Jesus' healing powers by restoring the ear of a high priest that had been cut off by one of those swords. In doing so, he capitulates magnanimously, where he could have instead used force to resist arrest.

That very part was to fulfill the scriptures written by Isaiah which he cited.

Isaiah 53:12 (KJV)
¹²  [...] because he hath poured out his soul unto death: and he was numbered with the transgressors; and he bare the sin of many, and made intercession for the transgressors.

Link it also to the fact that he was on the cross in the middle of 2 robbers.

Now, the high priests seek to trick Jesus with questions so they could incriminate him, but they had no reason to. A crime had to be committed before they could at least justify their arrest in the garden.

Luke 20:20 (KJV)
²⁰  And they watched him, and sent forth spies, which should feign themselves just men, that they might take hold of his words, that so they might deliver him unto the power and authority of the governor.

The emphasis on the swords could not be placed out of context to mean Jesus condoned violence. Jesus tells the disciples to sell their cloak for a sword showing how urgent a sword was needed at that particular time in the garden. Two swords that were shown to him obviously could not fight off the armed soldiers who came for his arrest, but he said they were enough.

Luke 22:52-53 (KJV)
⁵²  [...] Be ye come out, as against a thief, with swords and staves?
⁵³  When I was daily with you in the temple, ye stretched forth no hands against me: but this is your hour, and the power of darkness.

Luke 22:49 (KJV)
⁴⁹  When they which were about him saw what would follow, they said unto him, Lord, shall we smite with the sword?

Jesus did not answer "No!"; he did not stop what was about to happen. Lo and behold,

Luke 22:50-51 (KJV)
⁵⁰  And one of them smote the servant of the high priest, and cut off his right ear.
⁵¹  And Jesus answered and said, Suffer ye thus far. And he touched his ear, and healed him.

It was after this incident, according to John, that Jesus stopped the violence.

John 18:11 (KJV)
¹¹  Then said Jesus unto Peter, Put up thy sword into the sheath: the cup which my Father hath given me, shall I not drink it?

This meant that if Peter succeeded in defending Him, it would hinder His death, our salvation which God had planned.

Of course the disciples wouldn't understand Him, but a little crime (2 swords) was enough to (number Him among transgressors) to fulfill Isaiah's prophecy.