If you visited your Declare War page, you can see this little green button located under "Fight".
All the button does is to refresh your Declare War page to get new targets so you can go in war with them.
Besides the refresh button, there are also new reward items, achievements and timer update.
Click on the question mark button(?) on the Declare War page and you will be able to view the new achievements and rewards.
New Reward Items -
Click on the arrow sign(>) to view the Achievements page:
New Achievements -
You can also find the information on how to achieve the particular achievement through the Achievements page located in your Profile.
And lastly, the Timer Update!
We can now declare war once every 8 hours, i'm sure it's a pretty great update from Zynga :)