Since CO2 emissions are cumulative and persistent, and the United States has emitted more CO2 over the years than any other country, then your first sentence is directionally correct.  

The following figures from the World Resources Institute show the top 10 nations as measured by their cumulative emissions between 1850 and 2007.
1. US: 339,174 MT or 28.8%
2. China: 105,915 MT or 9.0%
3. Russia: 94,679 MT or 8.0%
4. Germany: 81,194.5 MT or 6.9%
5. UK: 68,763 MT or 5.8%
6. Japan: 45,629 MT or 3.87%
7. France: 32,667 MT or 2.77%
8. India: 28,824 MT or 2.44%
9. Canada: 25,716 MT or 2.2%
10. Ukraine: 25,431 MT or 2.2%