Remarkable progress made in quantum cascade lasers (QCLs) has led them to find an increasing number of applications in remote sensing, chemical sensing, and free space communications, in addition to potential space applications. However, little has been reported on reliability and failure modes of QCLs although it is crucial to understand failure modes and underlying degradation mechanisms in developing QCLs that meet lifetime requirements for space missions. Focused ion beam (FIB) techniques have been employed to investigate failure modes in various types of laser diodes. Our group has also used FIB to study failure modes in single-mode and multi-mode InGaAs-AlGaAs strained QW lasers, but few groups have used this technique to investigate failure modes in QCLs. In our study, we report on destructive physical analysis (DPA) of degraded InGaAs-InAlAs QCLs using FIB and high-resolution TEM techniques. The active region of QCLs that we studied consisted of two-23 stage layers of InGaAs-InAlAs separated by a 0.5 μm thick InP spacer layer for 8.4μm QCLs and 30-stage layers of lattice-matched InGaAs-InAlAs heterostructure for 4.7μm QCLs. The MOVPE-grown laser structures were fabricated into deep-etched ridge waveguide QCLs. L-I-V-spectral characteristics were measured at RT under pulsed operation. Our 8.4μm QCLs with as-cleaved and HR-coated facets showed a laser threshold of 1.7 A and a threshold voltage of 13 V at RT, whereas our 4.7μm QCLs without facet coating showed threshold currents of 320 - 400 mA and threshold voltages of 13 - 13.5V. Failures were generated via short-term tests of QCLs. FIB systems were used to study the damage area on the front facet and also to prepare TEM cross sections at different locations along the waveguide for defect and chemical analyses using a HR-TEM. In contrast to the COMD damaged area showing as a blister on the front facet of QW lasers, the damaged area of QCLs was significantly extended into the InP substrate due to a much less absorption of lasing photons in QCLs. Our detailed destructive physical analysis results are reported including defect, structural, and chemical analysis results from degraded QCLs.