Britain had guaranteed Belgian neutrality under an 1839 treaty. Sir Edward Grey warned Germany that Britain would declare war if Belgium was invaded.
Citation:
Editors. "Germany declares war on France, invades Belgium".
The Literary Encyclopedia. First published 01 March 2010
[http://www.litencyc.com/php/stopics.php?rec=true&UID=12204, accessed 21 May 2013.]