The four-nation tournament is being held in the Netherlands this week with World Cup semi-finalists Croatia taking on the hosts in the first semi-final in Rotterdam on Wednesday before Italy and Spain meet 24 hours later in Enschede.
The winners move into the final in Rotterdam on Sunday, which takes place after the third-place playoff.
Croatia's run to the 2022 World Cup semis burnished a reputation they forged four years earlier in Russia when they lost to France in the final, but both the Netherlands and Spain left Qatar believing they could have done more.
The Dutch exited the quarter-finals in a shootout while Spain were stunned by Morocco in the last 16.
Italy failed to qualify for 2022 but have allowed Roberto Mancini to stay on and try to rebuild the side. He will have five players from the Inter Milan side that lost Saturday’s Champions League final to Manchester City in a youthful squad.
"It's a great achievement to be back in the final four," said Mancini. "In the group we had good teams like Germany, England and Hungary. We're in the final stage and it's clear that we want to win, like the others, but it won't be easy."
Spain have undergone a major overhaul since the World Cup under new coach Luis de la Fuente, who made seven changes after their last set of internationals in March, including a recall for 37-year-old Jesus Navas.
The Dutch go into the tournament without striker Memphis Depay and hoping for a renaissance under Ronald Koeman after he replaced Louis van Gaal as coach.
They now have a chance to win a first major title after finishing runners-up at the World Cup in 2018 and third in Qatar in December.
This is the third edition of the Nations League, introduced to fill dates on the international calendar previously taken up by "meaningless" friendlies. Portugal won the first tournament in 2019 and France were champions two years ago.