Finland Parliament Website Targeted Ahead of NATO Entry: Cybersecurity Concerns

DoS attack made difficult to access the site, functions becoming temporarily unavailable.

Pro-Russian Hacker Group NoName057 (16)  Claims Responsibility for DoS Attack on Finland's Parliament Website Ahead of NATO Entry 

Finland becoming the 31st member of NATO, The country's membership was formalized through a series of steps taken in Brussels. 

Aki Luhtanen , said Russia’s war on Ukraine feels very close and NATO membership offers protection now and for the long term. 

What is the main purpose of NATO? What country left NATO?  Who was the first NATO country?  Who was NATO started by?