Since Israel's establishment, military expenditure constituted a significant portion of the country's gross domestic product, with peak of 30.3% of GDP spent on defense in 1975. In 2016, Israel ranked 6th in the world by defense spending as a percentage of GDP, with 5.7%, and 15th by total military expenditure, with $18 billion. Since 1974, the United States has been a particularly notable contributor of military aid to Israel. Under a memorandum of understanding signed in 2016, the U.S. is expected to provide the country with $3.8 billion per year, or around 20% of Israel's defense budget, from 2018 to 2028. Israel ranked 5th globally for arms exports in 2017. The majority of Israel's arms exports are unreported for security reasons. Israel is consistently rated low in the Global Peace Index, ranking 144th out of 163 nations for peacefulness in 2017.