Based on your reading of the following passage, what do you think is the most challenging aspect of combating terrorist attacks?
the 15 executive branch departments are following their national security strategy (nss) created to strengthen our national security. they are preparing for potential emergencies by investing in strong response teams, training, and materials. while these efforts are important, officials worry that terrorists will find ways around the defined strategies, since terrorists seem particularly good at changing the rules, adapting to current opportunities and recruiting previously unidentified supporters of terror.
intentional germ warfare, such as ebola
terrorists finding ways to avoid detection
nuclear weapon proliferation
attacks on our water systems

terrorists finding ways to avoid detection

Explanation:

This passage is explaining shorty and simply that the terrorists are always finding ways to avoid detection and to perform at least some of their planned activities. The terrorist organizations are constantly in search of new ways as to how to gain supporters and activists, be it some that will be directly included into the organization, or some that have became passively radicalized and start to work on their own for the cause. There are lot of ways of doing so with the modern technology, and that is always going in the advantage of the terrorists, and makes more and new problems for the anti-terrorist units in order to stop them.  

there are a number of different ideologies that come into conflict with regard to immigrants and immigration to the us.