By Daniel Michaels And Shayndi Raice 

FRANKFURT--German companies are on a buying spree--and the U.S. is their biggest target.

On Monday, pharmaceutical firm Merck KGaA said it would buy Sigma Aldrich Corp., a life-sciences company based in St. Louis, for $17 billion. A few hours earlier, German industrial giant Siemens AG agreed to buy energy-equipment maker Dresser-Rand Group Inc. for more than $6 billion.

The planned purchases are the latest in a flurry of U.S.-focused acquisitions by companies from Europe's biggest economy. The forces fueling the activity include record-low interest rates and stagnation in Europe, economic growth and declining energy prices in the U.S., and expanding cash hoards at thriving German companies.

"German companies want to be where their customers are," said Dietmar Rieg, president of the German American Chamber of Commerce in New York.

The latest deals bring to almost $70 billion the total value of German acquisitions announced in the U.S. so far this year, according to Dealogic. That ranks second to $77 billion in proposed takeovers by Canadian companies during a global M&A boom.

German firms have already spent more on U.S. investments so far in 2014 than in every full year of the past two decades, according to Dealogic.

Merck Chief Executive Karl-Ludwig Kley said Monday the Sigma Aldrich takeover marks "a quantum leap" for his company, in part because Merck has been "under-represented in the U.S." The acquisition also would let Merck tap into "trends like increasing globalization of research and pharmaceutical production."

Sigma Aldrich, which had sales of $2.7 billion last year, sells chemicals and biological materials used in scientific labs.

Merck KGaA's pharmaceutical sales are dominated by just two prescription drugs: Rebif for multiple sclerosis and cancer treatment Erbitux. Merck KGaA is unrelated to Merck & Co., although they shared common roots a century ago.

For Siemens, the Dresser-Rand deal advances Chief Executive Joe Kaeser's aim of building up the company's presence in the U.S. energy market and capitalizing on the shale-gas boom. He said Monday the acquisition would allow Siemens to be "seen and heard and known in the greater Houston environment," Dresser's home base and a center of the U.S. petroleum industry. Siemens has been expanding its energy operations and shedding other assets.

The German buying push in the U.S. highlights a confluence of economic trends. The biggest is macroeconomic: The U.S. economy is expanding, and economists expect that growth to accelerate. Europe has yet to rebound from recent financial crises, and now fears about tensions with Russia over the conflict in Ukraine could slow it further, analysts say.

Yoel Zaoui of London-based advisory firm Zaoui & Co., which advised Dresser Rand on its deal with Siemens, said the spate of deals shows how hot the U.S. market has become. "It's an open capital-markets environment and a growing economy," he said.

In a stark illustration of the contrast between the U.S. and Europe, the Federal Reserve is considering when to raise interest rates in the U.S., while the European Central Bank is seeking ways to cut record-low rates to stimulate growth. Germany's export-focused producers, which have thrived over recent years, are capitalizing on the situation.

"There's a friendly environment for M&A deals right now, given the big companies' high cash positions and given low interest rates," said Marco Gunther, an analyst at German bank Hamburger Sparkasse.

German manufacturers have also targeted the U.S. because the shale-gas boom is reducing American energy prices.

In Germany, meanwhile, energy prices are among the developed world's highest, due to the multibillion-dollar price tag for the government's decadeslong project to develop ecologically renewable forms of energy.

Industrial icons such as Siemens and BASF SE, the world's largest chemicals company, have said they plan to expand in the U.S. and other countries outside Europe, rather than at home in Germany, largely because of energy costs.

This week's deals fit in a broader push by German companies to grab technologies and market share in the world's biggest economy.

Last week, German auto-parts maker ZF Friedrichshafen AG agreed to acquire U.S. rival TRW Automotive Holdings Corp. for about $12 billion, while German software producer SAP SE said it would buy Concur Technologies Inc. of the U.S. for more than $8 billion. Both deals would bring the buyers new capabilities in critical future markets, such as self-driving cars and cloud computing.

"There is a little bit of a renaissance of focusing on U.S. acquisition targets," said Christian Kames, the Frankfurt-based head of German mergers and acquisitions at Citigroup Inc.

The spending splurge also shows a change of heart among German managers, said Dirk Albersmeier, head of German mergers and acquisitions at J.P. Morgan Chase & Co. in Frankfurt, which advised Germany's Merck on its deal Monday.

Many German executives had been wary of doing big acquisitions in the U.S. after past failures, he said.

With robust finances and strong positions in global markets, "it was just a matter of time" before deals restarted, he said. "Now, given the last couple of weeks, we have seen that people feel comfortable."

German companies have made a splash in the U.S. before--and not always successfully. When car maker Daimler-Benz AG in 1998 acquired Chrysler Corp. for $36 billion, then-CEO Juergen Schrempp promised to create "the most profitable automotive company in the world." But Chrysler struggled. In 2007, the German car maker sold most of its stake to private-equity firm Cerberus in a deal that brought Daimler AG almost no revenue.

Sports-equipment firm Adidas AG has posted three profit warnings over the past year, partly because its Reebok division, bought for $3.8 billion in 2005, has lost market share to rival Nike Inc. Adidas is now working to reposition Reebok.

But other German investments have boomed. Media giant Bertelsmann SE bought publisher Random House in 1998 for an undisclosed sum. It last year formed a joint venture with Penguin to create the world's largest book publishing group. Supermarket chain Trader Joe's has thrived since German retailing chain Aldi Nord acquired it in 1979.

The current series of deals also increases the likelihood other transactions will follow, as companies rush to grab assets before prices rise or rivals pounce. Rising stock prices both show and reinforce investor confidence, investment bankers say. Year to date, global M&A volume is $2.6 trillion, up 43% from the same period last year, according to Dealogic, and the most since 2007.

"The market environment in terms of financing and shareholder support continues to be positive," said Berthold Fuerst, the Frankfurt-based head of M&A in Germany for Deutsche Bank AG, which advised Siemens on its deal with Dresser Rand and ZF on its deal with TRW. "The ingredients remain for a positive environment."

Natalia Drozdiak and Sarah Sloat contributed to this article.

Write to Daniel Michaels at daniel.michaels@wsj.com and Shayndi Raice at shayndi.raice@wsj.com

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