Assumptions: are fictitious, costly

1 Chronicles 19:1-19
  King David sent some of his army to show comfort and condolences of kindness to a new king, whose father died.  This dead king had been kind to David and David desired to honor his memory.

  The advisors to the new king assumed David sent his men to spy out the land, to search and overthrow the new king.

  This “friendly” kingdom humiliated the men David sent to them.  The men’s beards were shaved and their garments cut off in the middle as far as their hips, then sent back to David… how humiliating!

  David heard how his men were treated and he went out to meet them.  Then David said, “Stay at Jericho until your beards grow, and then return.” 
1 Chronicles 19:5

  When the guilty men who did this humiliating act realized that “they had made themselves odious to David” – they paid an ally to come help them, against David and his army.  The troops gathered for battle against David.

  When David heard of this, he sent Joab (his general over his army) and all the army to confront the gathered troops, among whom were the kings of the various armies.

  Joab set up a battle plan which included his brother.  Joab’s plan was successful.

  David led his army and defeated the gathered army.  Remember, David relied on his commander in the field to do battle and be successful.

Consider this  ~

Assumptions are fictitious and costly.
Your assumptions may lead to war and defeat.