NATO is a military alliance between 29 North American and European countries.
Step-by-step explanation:
North Atlantic Treaty Organisation was formed in 1949 in the aftermath of World War 2.
It agrees on mutual defense of any country within NATO by all other countries in the alliance in case of an attack by any external powers.
From original number of 12 countries it has increased its fold to 29 countries in the present day.
It was initiated in the bid to fend off external powers from attacking any of the countries in the NATO.
Members agree to spend a target of 2% of GDP on defense by 2024.