Problem:
In John 14:27 Jesus says to His disciples:
Peace I leave with you, my peace I give unto you: not as the world giveth, give I unto you. Let not your heart be troubled, neither let it be afraid.
And in Matthew 10:34-36 Jesus says:
34 Think not that I am come to send peace on earth: I came not to send peace, but a sword.
35 For I am come to set a man at variance against his father, and the daughter against her mother, and the daughter in law against her mother in law.
36 And a man’s foes shall be they of his own household.
Solution:
In John 14:27 Jesus is talking to His disciples. So He is saying here that He gives peace to His followers.
In Matthew 10:34-36 Jesus is talking about the world, the unsaved people. God sends the sword, or warfare, to an earth of disobedient sinners.
In this chapter Jesus tells His disciples to go out into the world to preach the kingdom of heaven. So He was also warning them about what to expect from the world, and how even families become at odds with each other when they hear the gospel of Jesus Christ.