– Germany’s most difficult World Cup opponent might not be on the field in Russia.

The reigning world champions already have proved they can beat Mexico, the team they play in their group-stage opener, as well as Lionel Messi’s Argentina and Cristiano Ronaldo’s Portugal. They’ve played France, Spain and England to draws in the past seven months.

The foe Germany might have trouble overcoming is history.

Since 1938, only one country, Brazil (1958 and ’62), has won back-to-back World Cups, something the Germans will be trying to do this summer. That makes the targets on the backs of their uniforms especially heavy, says midfielder Ilkay Gundogan.

“Over the last three years, every team that we played against, they were just trying to beat the world champions,” he said. “That’s what makes it really hard.

“There are more contenders than just Germany. But of course we are the champions. And hopefully we’re going to be able to defend the title. That is our challenge.”

Germany returns nine players from the 2014 world championship team, but many of those missing were key contributors, among them captains Bastian Schweinsteiger and Philipp Lahm; forward Miroslav Klose, the top goal-scorer in World Cup history; and forwards Mario Gotze and Andre Schurrle, who came off the bench to combine on the only goal in the last World Cup final.

Arsenal midfielder Mesut Ozil, who started all seven games in Brazil four years ago, soon could join that list because of a knee injury suffered in last week’s game with Austria.

“Those players, they were essential for the World Cup trophy,” said Gundogan, who missed the 2014 tournament because of a back injury. “The real challenge is not to replace these kinds of players. But in Germany, we have a lot of talent, so the potential is definitely there.

“It’s up to us to show it.”

Depth may be Germany’s strength. The team won the Confederations Cup — an eight-team, pre-World Cup tournament — in Russia last summer with what was basically a “B” team. But that, too, may be a bad omen; no Confederations Cup winner has repeated as World Cup champion.

“You can do it,” said former Argentine international Mario Kempes. “But doing it is difficult.”

Kempes scored a tournament-high six goals — including two in the final — to lead Argentina to a World Cup title in 1978. Four years later, Argentina was knocked out in the second round.

“We didn’t have the same mentality that we did in 1978. And we didn’t have the best players,” he explained. “Everyone plays with the objective of being champion. But it’s difficult to do.”

Four years ago, Brazil, playing at home, was a strong World Cup favorite, and its players wilted under the pressure, stumbling into the semifinals, where it was pummeled 7-1 by Germany.

Germany appears equipped to handle pressure, given the steady hand at the wheel.

Joachim Low has coached the national team since 2006, guiding it not just to World Cup and Confederations Cup titles but to the semifinals of three consecutive European Championships and a third-place finish in the 2010 World Cup. It is arguably the most impressive coaching résumé in international soccer history.

And even after the roster turnover of the past four years, the team Low will lead in Russia will be among the deepest and most experienced in the tournament.

“The best way for me to deal with [pressure] was just to focus from game to game,” Schweinsteiger said. “The coach knows what to do.”

Put it all together — a deep roster, seasoned players, the most successful coach in international soccer with experience in handling pressure — and it’s hard not to see Germany defying history by winning again.