Extra security checks will be enforced at airports and railway stations across Europe following the Brussels terrorist attacks. But a huge shift in security operations is unlikely as analysts believe most European governments are already doing nearly everything they can to prevent terrorism.
Many commentators have pointed to failures of intelligence agencies and a lack of screening at entrances to airports and underground stations as the factors that enabled terrorists to kill 31 people and injure some 300 in Brussels on Tuesday.
Yet experts do not expect the national and EU-wide reviews that have followed the attacks to yield major changes in security systems, as European nations continue their long-term battle to prevent periodic attacks inspired by the Islamic State extremist militia.  
Aviation security has been stepped up around the world since the September 11, 2001 attacks in the United States, when Al Qaeda members took control of four passenger airplanes. However, the increased safety measures have mainly focused on the security of flight operations. To ensure that no explosives get on board, passengers have their luggage carefully screened, are forbidden from taking liquids through security, and sometimes have to take their shoes off.  But security checks at the outer perimeters of airports in many cities are still lax.
Before they triggered explosives in the busy departure hall at the Brussels international airport on Tuesday, the terrorists did not need to pass through any security screenings, highlighting the difficulties of securing large transportation hubs.
But if security checks are expanded to the outer areas of airports, it will bring up a number of unresolved issues, such as the needs for additional space and money.  At Frankfurt airport, one of Europe’s busiest hubs, security personnel would have to screen 170,000 passengers, 80,000 staff and an unknown number of visitors each day, according to some airline consultants.
Many aviation experts also believe that sealing off airports with additional controls would not solve the problem.
Terrorists simply shift their attention to alternative targets if they are prevented from reaching high-profile locations.
This strategy was seen during the Paris attacks in November, when suicide bombers blew themselves up outside the Stade de France stadium after failing to get past security into the stadium where 80,000 football fans were watching France playing Germany.
In general, airport terminals are hard to secure, just like other public spaces such as train stations and shopping centres.
The best way to meet terrorist threats is to improve intelligence sharing and better co-ordination by security agencies globally. Terrorism is not  a regional phenomenon, it is an international challenge and needs a joint strategy and response to counter it. Also, there should be an initiative  to tackle the root causes of terrorism.