To open the floodgates

To open the floodgates - if an action or a decision opens the floodgates, it allows something to happen a lot, or allows many people to do something that was not previously allowed.

Example - "Officials are worried that allowing these refugees into the country will open the floodgates to thousands more"

Image - http://www.quotehd.com/imagequotes/authors36/tmb/ron-peterson-quote-and-that-would-open-the-floodgates-to-many-other-si.jpg