Article 5 of the North Atlantic Treaty says that an attack on one member of the alliance is an attack on all members of the alliance. It is the reason why so many Eastern European countries aspire to join NATO. It offers security and peace of mind to people that were subject to Soviet occupation. One of those countries that rely so heavily on NATO is Estonia. The small Baltic state of about 1.3 million people borders Russia and is one of only 5 NATO allies that actually meet their 2% GDP defense spending commitment.
The election of Trump has shook Estonians who now fear they will be defenseless in the event of a Putin offensive. This article in POLITICO details these fears and what leaders are planning to do to work with Trump to maintain the alliance.