An integrated circuit (IC) Supply Chain Hardware Integrity for Electronics Defense (SHIELD) is envisioned to enable advanced supply chain hardware authentication and tracing capabilities. The suggested SHIELD is expected to be a ultralower power, minuscule electronic component that is physically attached to the host IC. This paper focuses on two important adversarial acts on SHIELD: physical reverse engineering and physical side-channel analysis. These attacks can be launched through mechanical or optical means and they can reveal and/or modify the confidential on-chip data or enable reverse-engineering of the design. For detection of these attacks and subsequent erasing of the sensitive data, sensors, erasure devices, and the relevant control circuitry need to be added to the SHIELD. We describe the device-level operation of the optical (photodetectors) and mechanical (nano- or micro-electromechanical switches) sensors and how they can be integrated within an IC to detect physical attacks. The operation of these micro/nano-scale sensors is unreliable due to environmental, operational, and structural fluctuations and noise. We outline system-level approaches to design a reliable countermeasure against physical attacks using unreliable sensors.