The competition was created to replace friendly internationals that were proving unpopular with clubs, players and fans as they interrupted league action.
To be held every two years, it features all 55 of Europe's national teams divided into four leagues that are themselves split into four groups of three, to be played between September and November.
Apart from promotion, relegation and financial bonuses in every league, the top one, which includes France, Spain, England and Germany, will finish with a four-team mini-tournament in June next year, to decide the champion.
Initially greeted with scepticism, the Nations League largely got the thumbs up after the first batch of matches.
"I really like the competition," said Germany coach Joachim Loew, whose team drew 0-0 in their League A clash against France last Thursday. "Because we have matches on a very high level."
"You could also see that the fans were really behind it," Loew said after almost 80,000 poured into Munich's Allianz Arena for the game.
The Germans were desperate to make amends for their World Cup fiasco in Russia that saw them crash out at the group stage in June, and few teams would be better suited than the new world champions.
There was equal enthusiasm in other top matches including in World Cup semi-finalists England's 2-1 loss to Spain on Saturday, with 81,000 spectators at Wembley Stadium.
"We have got to go through those games, we have got to go through those experiences," said England coach Gareth Southgate, who made only three changes to the team that played in the World Cup semi-final against Croatia.
"Otherwise... if we were just playing qualifiers now, against a lower standard opposition on the back of the result in the summer, we might have a perception of where we are which is false".
TOP-LEVEL MATCHES
For European heavyweights the Netherlands and Italy, the Nations League could not come sooner as a return to competition was important to both sides after failing to qualify for the World Cup.
The Dutch suffered a 2-1 loss to France on Sunday but were visibly content to be mixing it up again with Europe's big boys.
"I think we are on the right track," said Dutch coach Ronald Koeman.
But it was not only the big names that shone, with European minnows getting their chance to showcase their skills and potentially qualify for the European Championship in 2020.
Luxembourg enjoyed a sensational Nations League premiere, crushing Moldova 4-0 for their biggest win in 70 years while the Faroe Islands won 3-1 in their League D island derby against Malta.
Not everyone had the same intensity, though, with European champions Portugal resting Cristiano Ronaldo against Italy later on Monday and having sold 45,000 tickets for the Estadio da Luz which holds 64,000.
Asked whether his players were nervous, Portugal coach Fernando Santos said: "We have very experienced players, so there isn't that anxiety. Maybe if it was a World Cup, or European qualifying, there would be."
(Additional reporting by Martyn Herman, Nick Said and Brian Homewood. Writing by Karolos Grohmann; Editing by Christian Radnedge)
