Yes, Europe *can* defend itself without NATO. Whether it *will* is another question.
If the many countries of Europe are willing to co-operate, and devote the necessary resources, they can easily defend themselves against any likely near-term aggressor except the United States. They could even defend themselves against the US, although that would require getting a lot more serious about defence. Europe has the necessary population and economic resources, larger even than those commanded by the United States.
So yes, Europe can defend itself without NATO. However, I do not think it will be sufficiently motivated to devote the necessary resources, nor will it be sufficiently unified, to do so. The future obstacle to American hegemony will be Chinese, not European.
In a multilateral world diplomacy and politics are the tools required to prevent wars and resolve conflict. European countries have common objectives in these areas. The threats posed to Europe can be defended against by European initiatives acting together towards taking on these challenges. NATO was designed for the cold war and not for the issues that concern Europe in this era.
In a multilateral world diplomacy and politics are the tools required to prevent wars and resolve conflict, should a threat be posed to Europe a common defence initiative by Europeans are well able to keep all of Europe safe without the assistance of NATO. The cold war is long over and this military organisation does not serve Europe adequately with today's challenges.
Well, let's hope that it never comes to that kind of situation. Yes, I believe that Europe could still defend itself without the help of NATO. However, NATO does make such defense easier for them. The various independent military operations of European nations would do well in defending their own borders and interests.
I think Europe could defend itself from just about anyone, except for the United States and China, without Nato - if they could actually come together and get over their centuries-old babbling nonsense and differences. They are a collection of relatively powerful countries who could be potent when combined in the right way.
Europe could defend itself without NATO because each country would have alliances with other nations. Surely Great Britain wouldn't leave France or Germany out to dry in a conflict. European nations learned from World War II and the Balkan conflicts that coalitions and alliances work just fine. If NATO needs to be dissolved, it can be without much of a fuss. However, it would help if the defense of Europe is under one centralized command such as NATO.
Europe has been known for its relaxed defense tactics. The country tends to not mettle in other countries affairs too much. Therefore, I think it would be able to form allies of some sort in spite of not having NATO. Europeans tend to have a strong sense of nationalism in which the people would likely fight to defend their country.
The resent Problems in the Ukraine and Georgia show they still need a strong central alliance with North America. America needs Europe to counter Russia and the middle east so America can focus on the Rise of China. If anything we need to expand NATO and form a super alliance with Europe, North America, and Oceania. Russia and China could not challenge the might of the EU, America, Canada, Japan, S. Korea, Australia, New Zealand, Singapore the Philippines , and Taiwan. Isolationism was tried by America before WW2 and it lead to WW2 and the cold war. If we each go our separate ways they will pick us of one by one.
First they came for the Ukraine, and I did not speak out—
Because I was not a Ukrainian.
Then they came for Taiwan, and I did not speak out—
Because I was not a Taiwanese.
Then they came for the Senkaku Islands and the Baltic States, and I did not speak out—
Because I was not a Japanese or Baltic.
Then they came for me—and there was no one left to speak for me.
NATO was principally formed to support the European countries against the might of Russia with American power. I think without the massive strength of the US military, Europe could never hope to fight a war with Russia, so the answer is technically no. However, should NATO no longer exist, it is hard to imagine the US not getting involved anyhow, if Russia attacked Europe.