Student or Learner
How should I understand the underlined?
Thanks in advance.
The best lessons to bear in mind are those learned in such costly fashion during the past decade of the “war on terror”. Deal with the source of the problem, not just its symptoms. Keep the moral high ground. And pick fights you can win.
To "pick" a fight is to start a fight. It is never good advice to start a fight that you will lose. So "pick fights you will win" means to choose your battles carefully.