Mmmm, then the US would be on struggle street due to sheer lack of military capacity.
In 1914 the US Army in the lower 48 had 3 infantry and 1 cavalry divisions, only reorganised as divisions in peacetime in 1911 and with a peacetime personnel strength of 65 men per infantry company rather than the war strength of 150. The also had only 6 artillery regiments; 4 of field guns, 1 of horse/mountain guns and 1 'heavy' regiment with a battalion (2 batteries of 4 guns) of 4.7" guns and another of 4.7" howitzers. The US Army Regular did conduct a 'concentration' (mobilisation of the Regulars) on the Southern border in 1914, but it was a shambles; slow, disorganized and the orders were needlessly complex.
The NG was reorganised into divisions in 1912-13 and the law changed in 1903 and 08 to stipulate that the NG must be Federalised before the Federal Government could all for volunteers like they did in the Spanish-American war. However states neglected artillery in favour of far more useful infantry and cavalry, SecWar 1911-13 Henry Stimson said in his annual report said that it would take a year to train the NG into a force capable of fighting an intense war.
While it mightn't seem much and from the political wrangling it might even seem like nothing, but 1914-16 the US Army did a hell of a lot of the little things that set it up for the mobilisation efforts of 1917. They concentrated Regulars on the southern border, even moving the heavy regiment to the area and drastically improved the administrative aspects of the concentration (mobilisation in European parlance) orders. The NG also did a few things, while they accumulated some more artillery batteries they stood up battalion and regimental artillery HQs to make them more effective.
In 1916 the defence act authorised a bunch more Regular regiments, including 3 new artillery regiments (50% increase), and all 12 divisions (~170,000 men) of the NG was Federalised in support of the Punitive Expedition in July. The NG was still pretty woeful on an individual level, however with all 12 divisions mobilised the next 8 months were spent training them into a reasonably efficient fighting force in an environment where raiders were crossing the border into the US and the Regular Army was on the offensive deep in Mexico. It was this experience that allowed the AEF to expand so quickly.
I think any 1914 offensive into Mexico would be 'formation pigpile', pretty inept and possibly with heavy casualties as the US Army implemented in war the reforms it implemented over 2 1/2 years in peacetime. But, and this is a pretty big but, once they did so by 1915 they'd be a pretty formidable force and one that may have deterred Germany from undertaking USW.