What if the US's entry is delayed long enough for the Central Powers to consolidate their position, whilst the Entente are weaker, perhaps have an earlier Brest-Litovsk? Maybe if the Zimmermann Telegram wasn't intercepted by the British. Whilst it is highly unlikely that Mexico would accept the deal (they lacked the military might to fight the US, the means to integrate the new territories, would ruin their relations with much of Latin America, etc.) it might make the case for war harder for Woodrow Wilson to sell to the American people and Congress.[/QUOTE
Best bet is probably to have the Russian Revolution come a bit sooner - say in Oct-Nov 1916. That gives Bethmann a stronger argument against USW, which may well be delayed. It's also possible that Russia drops out more quickly, as the Revolution has come soon after the failure of the Brusilov Offensive, so that the troops are in a more negative mood, and the Revolution is immediately followed by the onset of winter, so that soldiers and civilians alike are more miserable and discontented than they were OTL between March an Nov 1917. In short, the Provisional Government has a harder row to hoe, and probably collapses quicker. If Russia is clearly leaving the war by say March 1917, then you may well not get USW at all, so no US intervention.