Not to mention that the United States may also get involved sooner because we are closer linked to Britain and France rather than the CP, so we might actually declare war after Lusitania.
In this case, the longer the Entente holds the lines, the CP becomes weaker and the Entente grows stronger. If the Entente make it to 1917, they will win, because the US will be fully for the war and AH and Russia may no longer exist as an effective fighting force. Will continue my mini-TL tomorrow!
I don't think that the US will declare war in this scenario, although everyone seems to think it will. The CP will be way too powerful for the US to risk getting involved. The US hesitated to join a war against Germany, AH and the Ottomans. Substitute the Ottomans for Russia is a BIG difference.