NATO Secretary General Jens Stoltenberg steps down on Tuesday after 10 testing years for the Western military alliance’s unity and purpose. Stoltenberg will hand over to former Dutch prime minister Mark Rutte with a wreath-laying ceremony and an official handshake at NATO headquarters in Brussels. Ambassadors from NATO allies will then meet in the alliance’s decision-making body, the North Atlantic Council (NAC), to appoint Rutte. The new Secretary General is expected to start his tenure with an address to NATO staff and a press conference. The changing of the NATO guard takes place as war rag…