While rolling out their new biometric entry system, popular countries in Europe have also tightened security. Here's what to know about their stricter rules.

By Philip Blenkinsop BRUSSELS, Jan 21 (Reuters) - U.S. customs authorities will face a real challenge if President Donald Trump goes through with his threat to place tariffs on six EU countries - rather than the whole European Union - given the ease of movement of goods between EU members.

After the latest bout of trade turmoil with the United States, European leaders are looking for ways to project strength to the Trump administration, which considers them weak.