Come 2021, and you may need to obtain ETIAS to visit the European countries. Of course, the requirement will be limited to a few areas of the region and would not be needed for entire Europe.
ETIAS, if you are not aware, is the new visa regime being introduced by the European Union. The formula was announced in 2018 and will come into force with effect from 2021. This will be applicable to the countries that did not need any Visa for visiting any countries in Europe. The new regime will now require the nationals from these countries to undergo a pre-travel screening. This is being done to avoid migration risks and other security concerns.
The regime is applicable to the 61 countries that have so far been enjoying a Visa free travel options and the Schengen Area which is a group of 26 countries that do not have an internal border between them. This may include Britain as well after the Brexit.
As of now, US citizens can travel to any country within the European region without any Visa and can stay there for a period of 90 days. However, the ETIAS regime is set to change all that. If you are a US citizen and looking to travel to Europe, you will need to obtain the travel authorisation in the form of ETIAS. Do note that the documentation is a travel authorisation and not a Visa as such. The ETIAS stands for European Travel Information and Authorization System and is planned for taking care of the abnormal migrant influx, acts of terrorism and other issues related to an uncontrolled entry of tourists in to the region that can jeopardise the infrastructure to a considerable extent.
If you are planning a visit to the European countries and belong to one of the countries that come under the Visa free concept that is currently being followed, make sure you opt for the ETIAS quite soon enough. There are several agencies that should help you get your ETIAS authorisation easily.