SHIELD (an acronym for Strategic Homeland Intervention, Enforcement, and Logistics Division) is an international defense organization with both covert and military branches. It specializes in defending Earth against abnormal threats, such as alien invasions.
SHIELD has a heirarchical command structure. The highest-ranking member of the organization is the Director, currently Nick Fury. Above the director is an international oversight council with representatives from the United States, United Kingdom, Russia, and China.
The sources of SHIELD's funding aren't clear, but the organization has a prodigious budget capable of building multiple helicarriers with full airwings in addition to fielding numerous intelligence agents, operating a substantial military force, and funding major electronic surveillance operations.
SHIELD was founded in the aftermath of World War II by Colonel Chester Phillips, Peggy Carter, and Howard Stark. Former Nazi/Hydra engineer Arnim Zola was brought into the organization as a technology consultant.
By the 21st century, SHIELD had become the preeminent international security organization in the world, specializing in threats posed by emerging "super heroes", advanced technology, and alien events.
Unfortunately, with the recruitment of Zola, SHIELD became infiltrated by Hydra agents from its inception. In the 21st century, the infiltration at SHIELD's highest levels became sufficient for Hydra to stage a coup for direct control of SHIELD's major centers of operations, including its helicarrier fleet. The coup was defeated by Captain America and Agent Natasha Romanov, but the revelation that SHIELD had been compromised to such a severe degree left the organization discredited. Labeled a terrorist organization by the US government, SHIELD no longer has any openly acknowledged agents, and its funding has been cut off.
Prior to the Hydra coup, SHIELD was a credible threat to virtually any nation on Earth. It was the primary defense against threats from other worlds, but even with its cutting-edge technology and resources, it dod not appear to be a match for a major interplanetary invader.
Following the coup, SHIELDs remaining agents are scattered and hiding. Many of the artifacts it had collected were stolen by Hydra agents and remain in hostile hands.