Just like NATO, the European Union has its own mutual defence clause.
Yet Sweden and Finland's decision to join the transatlantic alliance in the wake of Russia's war in Ukraine appears to show that trust in the bloc's version is flimsy.
The EU's Mutual Defence Clause — Article 42.7 in the Treaty of Lisbon — was approved in 2007 and has been in force since 2009.
It states that "if an EU country is the victim of armed aggression on its territory, the other EU countries have an obligation to aid and assist it by all means in their power."
It came 60 years after the creation of NATO and its collective defence clause — Article 5 — which provides that "an attack against one Ally is considered as an attack against all Allies".
Both have been triggered only once in reaction to terrorist attacks — 9/11 for NATO and the November 13, 2015 attacks in Paris in the case of Article 42.7.
EU's clause stronger?
"On the face of it, they look very similar," Aylin Matlé, research fellow in the Security and Defence Programme at the German Council on Foreign Relations think tank, told Euronews. "But in fact, the wording of Article 42.7 is much stronger in my opinion in comparison to NATO's Article 5."
The reason is the word "obligation" which implies that other EU member states must provide assistance of some sort. Yet, "that doesn't mean that anything has to follow, that military action has to follow automatically," Matlé pointed out.
Article 42.7 stipulates that while the obligation of mutual defence is binding on all EU countries, "it does not affect the neutrality of certain EU countries and is consistent with the commitments of EU countries which are NATO members."
This means that the type of assistance provided, if any, is still up to the political leadership in individual member states.
This, too, is somewhat similar to NATO.
Article 5 states that any ally will "in exercise of the right of individual or collective self-defence" take "individually and in concert with other parties, such action as it deems necessary, including the use of armed force."
EU's clause seen as 'politically weaker'
Twenty-one EU member states are part of NATO and a third of them — Belgium, Denmark, France, Italy, Luxembourg, the Netherlands and Portugal — are founding members of the transatlantic military alliance.
The remaining ones joined in the following decades with the last one, Croatia, officially becoming a member months before the Treaty of Lisbon entered into force.
Sweden and Finland, the latter of which shares a 1,340 kilometre-long border with Russia, could soon join the fold. Public opinion on joining the military alliance shifted dramatically in the two traditionally neutral countries in the weeks after Russia launched its unprovoked attack on Ukraine and both have now formally put in membership bids.
Both are EU member states and are thus theoretically covered by Article 42.7.
According to Rafael Loss, coordinator for Pan-European Data Projects at the European Council on Foreign Relations (ECFR) despite its strong rhetoric, Article 42.7 "is considered by most EU member states, and a lot of them are members of the NATO alliance, as politically a bit weaker."
"There is sort of general, at least implicit, agreement that NATO is in charge of territorial defence in Europe and the EU does crisis management to some extent and this is not exclusive of course," he added. The fear in Stockholm and Helsinki, Loss said, is that if Russia were to attack, the EU's response would simply be "much less than what NATO is capable of".
NATO is military alliance only
The difference between the two polities is that NATO is a military alliance only with regular joint exercises as well as multinational battlegroups and important capabilities including fighter jets and warships under direct NATO command already deployed across Europe.
The EU, however, was created as a political and economic alliance and is only just really starting to sketch common security and defence infrastructures.
This process was accelerated by the war in Ukraine with leaders backing the Strategic Compass policy in late March that plans for the establishment of an EU Rapid Deployment Capacity of up to 5,000 troops, more regular live exercises on land and at sea and the possibility for member states to jointly invest in the development of military capabilities.
The Commission also unveiled last month proposals for an EU framework for joint defence procurement to allow member states to quickly and more cheaply replenish stockpiles sent to Ukrainian authorities, replace Soviet-era equipment and plug military capability gaps, especially air and missile defence systems.
According to Matlé, Sweden and Finland used France's invocation of Article 42.7 in 2015 as a test case, especially given that neither was seriously considering NATO membership at the time.
"Those two countries were actually very keen on sort of not only the invocation but also moving ahead and moving the EU's common security and defence policy further. And I guess it's you know, you could argue that the balance sheet of the EU’s common security and defence policy is sobering," she added, highlighting the lack of a common command structure and lack of military capabilities at the EU's disposal.
Who if not France and Germany?
The other issue is leadership.
NATO is dominated by the US which has tens of thousands of troops — some under NATO command, some under bilateral agreements with individual EU member states — dotted across Europe as well as significant capabilities.
Some smaller eastern countries have been wary of giving the EU a strong defence mandate, fearful that this might lead to a pull-back by Washington, especially as the Union tends to be slow to respond to crisis due to the unanimity requirement.
The EU has imposed sixth rounds of sanctions on Russia since its invasion on February 24 but huge cracks in the bloc's unity appeared during negotiations for the latest package which took four weeks.
The amount of military aid each member state has also provided Ukraine has varied considerably with Germany criticised heavily for its initial slow response while French President Emmanuel Macron has profoundly irked eastern counterparts with his call not to humiliate Russia.
"The major question that we're facing in the EU context is that there has been a significant drop in trust vis-a-vis the German and French governments because they're not leading the Western response to this war and in fact, are hesitating," Loss said with the US, UK and eastern European countries including Poland and the Baltics seen as the ones providing Ukraine with the support it needs to defend itself.
"This is exactly what is currently sinking European support for any idea of, you know, European defence sovereignty because who would lead such an effort if it's not France and Germany that at this moment are failing to pull their weight?," he concluded.