Leaders from 28 NATO countries are set to approve a new strategic concept at their Lisbon summit this week in a bid to tackle the alliance's existential crisis in the wake of the end of the Cold War.

Since the former Soviet Union and the Warsaw Pact were dissolved in the early 1990s, NATO, established during the Cold War, lost its original purpose and was caught up in an identity crisis, following debates over whether the regional military organization should continue to exist or not.

Amidst doubts, NATO started its journey of redefining and reorienting itself to keep pace with the fast-changing global environment.

In 1991 and 1999, NATO successively unveiled two "Strategic Concepts," which serve as general guidelines for the bloc's development.

Under the concepts, NATO believes that the risk of all-out war in Europe has disappeared but members are endangered by threats of ethnic conflicts, abuse of human rights, political instability, economic fragility and proliferation of weapons of mass destruction.

The concepts assert that NATO will remain as the cornerstone of trans-Atlantic security and should stick to its policy of collective defense to guarantee members' security.

NATO began to adjust its strategy and focus on conflicts prevention and crisis management by launching military operations beyond its borders to show its strength.

In 1995, NATO carried out its largest military operation to date, having forced Serbia and the former Yugoslavia to sign the Dayton Agreement. In 1999, NATO launched a bomb attack in the former Yugoslavia without the authorization of the U.N. in its first assault on a sovereign state since it was established.

After the 9-11 attacks in 2001, NATO made fighting terrorism a priority and took part in the war in Afghanistan.

Furthermore, NATO decided to cooperate with "former rivals."

Under the bloc's open-door policy, dozens of countries joined NATO, including states in Eastern Europe, the Baltic Sea region and the Balkans.

As a result, NATO's membership swelled from 16 at the end of the Cold War to its current 28.

The eastward expansion of NATO has triggered strong protests from Russia, which showed its opposition to the bloc's growth through the war with Georgia in 2008.

NATO's military operations and eastward expansion raised concerns over the alliance flexing its muscles to act as a "global cop."