Short Answer: Roe v Wade.
Long Answer: 40 years of exporting our birth control policies through foreign aid agreements.
Since 1973, USAID has followed the Helms Amendment ruling, banning use of US Government funds to provide abortion as a method of family planning anywhere in the world (Wikipedia).
However, we know that the Helms Amendment does not cover other birth control.
The Helms Amendment was embodied in the Mexico City Policy, "an intermittent United States government policy that requires all non-governmental organizations (NGOs) that receive federal funding to refrain from performing or promoting abortion services as a method of family planning with non-US government funds in other countries." (Wiki)
History of the Mexico City Policy
1984 - President Reagan passed the Mexico City Policy.
1993 - President Clinton rescinded the policy
2001 - President George Bush reinstated it.
2009 - President Obama rescinded the policy.