U.S. plans to arm Ukraine infuriated Russia and aggravated deteriorating relations, but a bigger worry is the weapons falling into enemy hands—as has happened before.

Washington last month decided to send lethal aid to Ukraine to prevent aggression by pro-Russia separatists. The move is meant to keep the simmering conflict from flaring into a hot war by improving government forces’ ability to defend against possible rebel offensives.

But the policy is fraught with potential national-security pitfalls, particularly the risk of the weapons ending up on the other side.

U.S. officials say they have carefully considered the dangers, taking particular precautions with Javelin anti-tank missiles the Trump administration is sending.

Those weapons have been designated for units far back from the eastern Ukraine frontline to ensure that Ukraine doesn’t use them for an offensive push and they aren’t lost to the enemy if the fighting shifts.