From sundown last night, until sundown tonight, Israelis are fasting in remembrance (among other things) of the destruction of both the first and second temples.
Wikipedia has as a pretty good article on the topic, here.
It really is one of the most significant days of the year, and it directly impacts Israeli nationalism. And, in its own way, it reinforces the national identity of Israel and reminds everyone of the mistakes of the past and the hope for the future.
Tisha b’Av is a reminder to us all.