For the seed which one day produces the reward which righteousness brings can only be sown when personal relationships are right and by those whose conduct produces such relationships. (James 3:18; William Barclay's Daily Bible Study)
In this verse, James is talking about a social situation. God's purpose - the fruit that He wants from His way of life, the kind of character that He wants in us - has to be produced in peace. It cannot be produced in war.
Why it cannot be produced in war is obvious. When one is involved in war, he is thinking only of himself, which runs 180 degrees counter to God's nature. God's nature is outgoing. When one is engaged in war, all one is seeking to do is to preserve the self. For God's purpose to be fulfilled to the very best degree, peace is required.
The seed, which one day produces the reward that righteousness brings, can only be sown when personal relationships are right, and by those whose conduct will produce such relationships.
Jesus says that peacemakers will be the children of God, not those who butt others aside, aggressively trying to get to the top, asserting themselves, their will, and their ideas in every circumstance, angling to be the big shot. "Out of my way, buddy. That is my beat." Those people, by implication, will not see God.
This is why God will permit a divorce. Does He not say through Paul in I Corinthians 7:15, "If the unbeliever departs, let him depart"? The believer "is not under bondage in such cases" because "God has called us to peace." God will permit a divorce so that a person can be saved due to the subsequent peace. In a family in which a war rages between a husband and wife, it is possible that God may lose both of them.
When those who butt and disturb the flock are present, the flock will not prosper. The shepherd has to ensure that there is peace, freedom from fear from the outside, freedom from tension within, and freedom from aggravation. (We even use the term "bug," which is what insects do to sheep: They irritate them to no end so they cannot gain weight and are discontented.) The shepherd must also make sure there is freedom from hunger - a congregation, a flock, will prosper if it is being well-fed.