Forced together by fears of a Russian cyber attack, a migration crisis and failing states in Europe's neighbourhood, the challenges before NATO and EU require both a military response and a softer security approach, combating propaganda and providing training to stabilise governments.
But while 22 of NATO's 28 members are also members of the EU, both institutions face limits on what they can do together because of territorial tensions between Turkey and Greece that limit information sharing.
"We want to be ready to respond and coordinate our actions if and when one of our members or institutions comes under hybrid threats. We equally want to prevent attacks," the head of the European Council, Donald Tusk, said.
"We have a united stance on Russia. The EU has delivered economic sanctions, NATO has delivered the greatest increase in our collected defense since the end of the Cold War," NATO secretary general Jens Stoltenberg said.
Despite the signing a pact, problems could arise with a reluctance to undertake missions without United Nations' support, which Russia, a UN security council member, is wary of granting with east-west ties at their lowest point since the Cold War over the crisis in Ukraine.