Preah Vihear is the subject of a long-running territorial dispute between Thailand and Cambodia, and several soldiers on both sides were killed in clashes in 2008, 2009 & 2011. Both sides are trying to work out a diplomatic solution.
Land mines remain a real danger in the area, although the temple itself and the access paths have been painstakingly cleared by the HALO Trust. Stay on the beaten path, don't venture into any vegetation which has not been cleared recently, and heed the red warning signs, painted rocks and strings marking the limits of the demined area.
The cliffs are steep and no provisions are made to protect you from your own carelessness. Keep a very close eye on children.