If you follow the link provided above it shows that your Roth IRA contribution might be limited based on the MAGI which includes the military pension. To calculate your modified adjusted gross income, take your AGI and add back certain deductions. If you are over a certain MAGI then you cannot contribute to a Roth IRA.
You are mixing it up with the annual limit, these are two different criteria:
For 2018, your total contributions to all of your traditional and Roth IRAs cannot be more than:
$5,500 ($6,500 if you’re age 50 or older), or
your taxable compensation for the year, if your compensation was less than this dollar limit.
