Defense ministers of the North Atlantic Treaty Organization (NATO) ended their two-day talks in Brussels on Thursday whose agenda was dominated by discussions about increasing defense spending.

The issue became more consequential as U.S. President Donald Trump called for fair burden-sharing.

Ahead of the talks, NATO Secretary General Jens Stoltenberg briefed the press Tuesday on the agenda, stating that burden-sharing would be a priority point of discussion and that the organization's "continuous adaptation requires responsibilities to be shared fairly among allies."

During a 2014 summit in Wales, NATO members made a commitment to gradually increase defense spending to reach a target of 2 percent of GDP expenditure on defense in a decade.

In 2016, the defense spending increased by 3.8 percent, or roughly 10 billion U.S. dollars, among European NATO allies and Canada, which was "significantly higher than what we had originally foreseen," said Stotlenberg at the press briefing.

"This makes a difference but it is absolutely vital that we keep up the momentum," he added.

Currently, only five out of the 28-member organization have reached the 2-percent target, namely, Estonia, Greece, Poland, Britain and the United States. In total, 22 members saw an increase in defense budget in 2016.

These defense spending shortfalls have been a source of criticism for Trump's administration, with Trump himself telling a German newspaper last month that he felt that NATO was "obsolete," not dealing effectively with terrorism, notably because "countries are not paying what they should."

The change of stance towards NATO by an American president alarmed many members who agree that NATO is facing the greatest challenge to its security since the end of the Cold War.

Increased and more varied security threats have led some to wish for an even stronger commitment from the United States, but recently appointed U.S. Defence Secretary James Mattis made it clear that without more equal burden-sharing, Americans might withdraw some of their support from NATO.

"America will meet its responsibilities, but if your nations do not want to see America moderate its commitment to this alliance, each of your capitals needs to show support for our common defense," said Mattis on Wednesday in Brussels.

For the retired U.S. general and former NATO Supreme Allied Commander, it was no longer acceptable for U.S. taxpayers to carry a "disproportionate share of the defense of Western values."

"Americans cannot care more for your children's future security than you do," Mattis said. "Disregard for military readiness demonstrates a lack of respect for ourselves, for the alliance, and for the freedoms we inherited, which are now clearly threatened."

The United States' ultimatum arrives at a difficult time, not only because of the diverse security threats NATO potentially face, but also due to the long-term budgetary difficulties under which many members have been struggling since the global financial crisis.

Several countries were under pressure to increase their defense spending at a faster pace than reaching the 2-percent target by 2024.

According to analysts, this would risk bringing countries into conflict with the European Union (EU)'s fiscal rules that require them to continue reducing budget deficits.

With the probability of the U.S. role in European security diminishing, debate has been resumed on the continent about the creation of a European Defence Union to complement both NATO and their national defense forces.

Leading the call for a European Defence Union and the establishment of a headquarters for defense research and operations has been European Commission President Jean-Claude Juncker, who has rejected Trump's political pressure on the EU.

"It has been the American message for many, many years. I am very much against letting ourselves be pushed into this," said Juncker on Thursday at a security conference in Munich.

"Europeans must bundle their defense spending better and spend the money more efficiently," he continued.

The efficient use of funds for an EU defense force, however, is far from reaching consensus, with many worrying that it would double and conflict with NATO. Others argued that an EU military, rather than a coalition force of national militaries, would turn the union toward even greater integration.

source: Xinhua