Stephen Strasburg finished 13th in the majors with a 3.00 ERA last season. (David Goldman/AP)

Last season, the Nationals had an above-average pitching staff. Stephen Strasburg finished 13th in the majors with a 3.00 ERA. Jordan Zimmermann played in his first All-Star game. And Gio Gonzalez had a sub-3.40 ERA for a fourth straight season. Not a bad year for the team’s top three starters. But wait until this season. Here are four reasons the Nationals will have the best starting rotation in baseball in 2014.

1. Strasburg unleashed

It’s time to let Stephen Strasburg loose. The Nationals have been understandably cautious since their ace had Tommy John surgery in 2010. He came back to pitch five games at the end of 2011, then had an innings limit and was shut down early in 2012. Last season, three years after his surgery, Strasburg pitched into the ninth inning just twice. Out of the top 30 pitchers in ERA last season, the Nats right-hander averaged the fourth-fewest pitches per start (95.0). He finished the 2013 season 8-9 with a 3.00 ERA, 191 strikeouts and 1.05 WHIP — pretty much top-of-the-rotation stats aside from his record. If Strasburg is able to stay healthy — he had a short stint on the disabled list last year for a strained muscle in his back — he should be in the Cy Young race. There’s no reason why the 25-year-old shouldn’t reach 200 innings for the first time in his career this year.

2. Big upgrade

Doug Fister will open the season on the DL with a strain in his right lat muscle, but if he’s able to get healthy he should be a big upgrade from Dan Haren — the Nationals’ No. 4 starter in 2013. Last year, Haren went 10-14 with a 4.67 ERA, while Fister was 14-9 with a 3.67 ERA for the Tigers. If Fister — with that 3.67 ERA — had pitched Haren’s 1692⁄3 innings in 2013, the Nationals would have allowed nearly 20 fewer runs and ranked third instead of seventh in the majors in starters’ ERA. Fister is also moving from the AL to the NL, where he won’t have to face a designated hitter on a daily basis. Lots of pitchers see significant improvement when changing leagues. For example, Gio Gonzalez’s ERA went from 3.12 in his last season with the A’s in 2011 to 2.89 in his first season with Washington in 2012. If Fister has even a slight improvement on his 3.53 career ERA, there won’t be a better No. 4 starter in the majors in 2014.

3. Prime time

The top four pitchers in the Nationals’ rotation will be from 25 to 30 years old and have four to six years of experience. Washington has a group of starters who are right in the prime of their careers. Since 2003, only four out of 22 Cy Young winners were older than 30; two have since been linked to performance-enhancing drugs (Roger Clemens and Bartolo Colon), one was out-of-nowhere knuckleballer R.A. Dickey and the last was Roy Halladay. Pitchers peak in their late 20s or early 30s — which could mean a career-best year for Stephen Strasburg, 25, Gio Gonzalez, 28, Jordan Zimmermann, 27, and Doug Fister, 30.

4. Defense has their back

The Nationals’ defense didn’t exactly help the pitchers last season. Washington had the seventh-most errors (107) in the majors. Infielders Ian Desmond, Ryan Zimmerman and Adam LaRoche each ranked in the bottom three in their position in errors. Zimmerman, who may see some time at first this year, seemed to fix his throwing problems toward the end of the 2013 season. LaRoche was a Gold Glove first baseman in 2012. And Desmond has shown flashes of being a top-tier defensive shortstop. At one point, he had 59 straight games without an error last year. The Nationals are not a bad defensive team, but they had a really bad defensive year in 2013. A sound defense would only help this powerhouse rotation.

Arms Race

Here are four teams whose pitching staffs could rival the Nationals’ dominant starting rotation this season:

1. Dodgers: With Cy Young winners Clayton Kershaw and Zack Greinke leading this staff, the Dodgers have the best one-two punch in baseball. But they lack depth. Good luck with Dan Haren as your No. 4 starter.

2. Braves: A promising young rotation had an injury-filled spring that required Atlanta to sign Ervin Santana to a one-year deal in mid-March. Kris Medlen and Brandon Beachy had season-ending Tommy John surgery earlier this month.

3. Tigers: Max Scherzer won the AL Cy Young last year and he’s Detroit’s No. 2 starter. The Tigers’ depth is comparable to the Nats’, with Justin Verlander, Scherzer, Anibal Sanchez, Drew Smyly and Rick Porcello.

4. Cardinals: Adam Wainwright is the veteran ace on a staff of talented arms. But they’re inexperienced. How will 22-year-old Michael Wacha fare in his first full big league season? And how will 23-year-old Shelby Miller follow up his stellar rookie year?