Abstract
The logic of proxy war suggests that states may delegate fighting to civil war parties when they want to avoid direct confrontation with their rivals. Yet, from Vietnam and Afghanistan during the Cold War to the contemporary cases of Libya, Ukraine and Syria, foreign states may end up intervening directly in support of their beneficiaries, deploying advisors, special forces, and even ground troops. Despite the countless number of studies on the use of force, including external intervention in civil war, we lack an understanding of why, and when, foreign sponsors would decide to directly intervene in a civil war by sending combat forces and not limit themselves to indirect support such as weapons and training. What drives the escalation of foreign support to direct intervention? This paper develops a logic of support escalation combining poor conflict evolution, foreign leaders' commitment, and the need for quick reversal of battlefield outcomes. Because there are several forms of escalation, sponsors will only provide direct support when (1) the costs of proxy war are about to rise significantly and (2) the proxy's forces lack combat effectiveness to obtain meaningful battlefield success. To compensate for the proxy's losses in a short time, foreign states may end up escalating the extent of their commitment, thereby abandoning the initial logic of limited war. The paper applies the logic of support escalation to a number of contemporary cases, before delving into the case of the Spanish Civil War and tracing the origin of Fascist Italy's increasing involvement via the analysis of primary documents. The paper further probes the generalizability of the argument with a large-N analysis of direct intervention in civil wars from 1975 to 2018.