An X-class solar flare occurring on 14 July 2000 (Bastille Day) marked the initiation of events which led to a powerful geomagnetic storm on 15 July. At the time of the flare the NASA-IMAGE spacecraft was near apogee and observed a global-scale transient increase in far ultraviolet (FUV) emissions of the thermosphere in both the N2 Lyman-Birge-Hopfield (LBH) band and the O I 135.6-nm line and also in geocoronal emissions of H I at 121.6-nm. Comparison with the solar x-ray flux measured by GOES 10 shows the delayed response of the FUV emissions compared to the peak in x-rays. This lag is attributed to a later peak in EUV emissions, which drive the photoelectron flux in the energy range which is, in turn, most effective in producing N2 LBH and O I 135.6-nm emissions. Measurements of solar EUV input by Solar and Heliospheric Observatory (SOHO) Charge, Element, and Isotope Analysis System (CELIAS)/Solar Extreme-Ultra-Violet Monitor (SEM) confirm the close correlation between solar EUV and terrestrial FUV emissions. A comparison of the flare-time variation in the signal in each channel of the instrument allows one to (1) determine the variation in resonantly scattered solar emissions and (2) differentiate between emissions created through photodissociation by solar EUV and those driven by terrestrial photoelectron impact. The latter provides a means for positively identifying the dayglow emissions observed in the SI-12 instrument.