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(Abridged) We report on new and archival X-ray and near-infrared observationsof the anomalous X-ray pulsar 1E 1048.1-5937 performed between 2001-2007 withRXTE, CXO, Swift, HST, and VLT. During its ~2001-2004 active period, 1E1048.-5937 exhibited two large, long-term X-ray pulsed-flux flares as well asshort bursts, and large (>10x) torque changes. Monitoring with RXTE revealedthat the source entered a phase of timing stability in 2004; at the same time,a series of four simultaneous observations with CXO and HST in 2006 showed thatits X-ray flux and spectrum and near-IR flux, all variable prior to 2005,stabilized. The near-IR flux, when detected by HST (H~22.7 mag) and VLT(K_S~21.0 mag), was considerably fainter than previously measured. Recently, in2007 March, this newfound quiescence was interrupted by a sudden fluxenhancement, X-ray spectral changes and a pulse morphology change, simultaneouswith a large spin-up glitch and near-IR enhancement. Our RXTE observationsrevealed a sudden pulsed flux increase by a factor of ~3 in the 2-10 keV band.In observations with CXO and Swift, we found that the total X-ray fluxincreased much more than the pulsed flux, reaching a peak value of >7 times thequiescent value (2-10 keV). With these recent data, we find a stronganti-correlation between X-ray flux and pulsed fraction, and a correlationbetween X-ray spectral hardness and flux. Simultaneously with the radiative andtiming changes, we observed a significant X-ray pulse morphology change suchthat the profile went from nearly sinusoidal to having multiple peaks. Wecompare these remarkable events with other AXP outbursts and discussimplications in the context of the magnetar model and other models of AXPemission.

X-ray and Near-IR Variability of the Anomalous X-ray Pulsar 1E 1048.1-5937: From Quiescence Back to Activity